UPDATE: I got an e-mail from Twomonkeysayoyo who put together a Pandora station based on this list. If you're interested in checking it out and having a listen, click here (Link will open in a new window).
A common criticism of metal bands, such as Metallica, is the fact that many of them end up "selling out" their harder edged sound for a more polished mainstream sound in the quest to be number one. Despite this, there are many bands out there that have never strayed far from their roots and remained true to their fans and their music.
It seems to be sad but true, but in the realm of heavy metal, many of the greats out there often turn to the "dark side" of commercialism or awkward changes in the quest for higher success and more riches. KISS, Metallica, Aerosmith, and Motley Crue have all been accused of selling out in the past, each of them bands that paved significant entries into the history of metal in their early days.
Sadly, the current scene has also diluted the effect and impact of metal. Turn on Headbanger's Ball and you will see hundreds of forgettable metal bands playing metal-by-numbers, in a systematic rip off of Iron Maiden riffs and Phil Anselmo screams. Additionally, the music industry continues to try to push the awful emo scene (Fall Out Boy, My Chemical Romance, AFI, etc.) as some kind of metallic resurgence...
At the end of the day though, metal heads can still turn to a few bands that continue to stomp true and keep metal alive worldwide:
Strapping Young Lad / Devin Townsend
When Devin Townsend's outfit Strapping Young Lad released their album City, it may not have hit number one, but it broke SYL out and made them more well-known in the metal community. Fans pondered what a follow-up could be like, some concerned it would be a little more polished. The result was far from it. Every follow-up SYL came out with was heavier, more brutal, more dynamic, or all of the above. Indeed, if you listen to any of the material Devin Townsend has created you will find it to be a worthy addition to your metal play list. Devy FTW.
Henry Rollins
After establishing himself as a solid frontman in the early punk scene with Black Flag, Rollins eventually moved on with his own band, the aptly named Rollins Band. While the early punk aspects were still there, Rollins smashed his way into the metal scene by making the band's material far more aggressive, intense, and brutal than anything he had done in the past. While the Rollins Band line-up has changed over the years, and the sound has evolved through different transitions, the sheer intensity and attitude Rollins presents with the band has stayed true since its inception. And the lyrics? Brutal honesty at its finest.
Slayer
Out of the biggest thrash acts that spawned from the 1980's and rose to major worldwide success, only one has stayed true to their sound, their image, and their fans from day one. Slayer has never wavered from their position of being a dark thrash metal band, and continues to deliver the goods to this day. Despite never having scaled the heights of success that bands such as Metallica achieved, Slayer continues to consolidate and strengthen their long term hardcore fan base, by being consistent.
AC / DC
Love them or hate them, you have to admit, AC / DC have never ever changed. They always have sounded like AC / DC, and will continue to do so in the future. The biggest challenge this band ever had to face was the death of the original vocalist, Bon Scott, who had a unique vocal sound and helped deliver some of the more memorable songs they recorded. Bringing in Brian Johnson as his replacement was a huge gamble on AC / DC's part in 1980, but it was a change that fans welcomed. Like them or not, AC / DC's impact on hard rock and heavy metal is unforgettable and indisputable.
Immortal
Hailing from Norway, the band members of Immortal never lost sight of their concept since beginning in the early 1990's. While they are often labeled as a black metal band, their sound is somewhat more of a multifaceted synthesis of classic thrash metal and traditional black metal. While most bands of the genre tend to focus on topics related to evil, the occult, or Satanism, Immortal were more inclined to delve into elements of darkness and winter, creating not only a unique tone for the band, but a fabulous image also.
Iron Maiden
While their years with Blaze Bayley as lead vocalist were not well received, Iron Maiden is one metal band that has remained consistent over the years. Come to think of it, is it even possible for Maiden to sell out? The mere thought of the possibility is too surreal. The Irons have continuously built upon their previous releases, and with the vocal air-raid siren that is Bruce Dickinson, the amazing guitar work, and the keen direction of bassist Steve Harris, the band has never failed to deliver.
Pantera
Dimebag Darrell is sadly missed to this day but his legacy remains in all of Pantera's major releases. Following the slow death of the popularity of hair metal, and during the dominance of grunge and the oversaturation of the alternative ideals, Pantera brought aggression, power, and solidarity back to the metal scene. Without the aid of heavy promotion and radio play, the band made a following for themselves out of word-of-mouth and constant touring; a grassroots effort that would eventually deliver confidence and determination to young metal heads around the world. After making a huge impact with 1992's Vulgar Display of Power, some fans wondered whether success would taint or commercialize the Pantera sound. The resulting albums that followed were far from any kind of sell-out: they were rawer, heavier, and more aggressive than ever. In the words of vocalist Phil Anselmo, the band's follow-up albums made people say, "What is wrong with them?"
Overkill
The godfathers of thrash are probably also the most underrated thrash metal band of all time. Formed in 1980 (well before Metallica was in its early beginnings), Overkill managed, and continue to manage, to release some of the most well-thought and finely crafted thrash metal albums of our time. Sadly, their evolution was always left in the shadows of the success of bands like Metallica, Megadeth, Anthrax, and Slayer, but they developed a very loyal following, that continue to support them to this day. You do not know thrash metal until you've listened to Bobby Blitz belt out his screams in Overkill.
Lamb of God
Out of the current metal scene, it is pretty hard to pick out which bands will continue for the long term, and which ones will or will not sell out. I would wager however, that there's a pretty good chance that Lamb of God will stay true to their roots, their sound, and their fan base. When Ashes of the Wake became their breakthrough album, the band could have easily toyed with the fruits of success and brought in some producer to polish up and commercialize their sound. Instead, the band went into the studio, became more aggressive, and developed an album called Sacrament; an album that debuted very high on the Billboard charts showing that not only metal was alive and well; but that bands like Lamb of God can continue to hit the bulls-eye without comprising the music. Walk with me in Hell...
Mortal Sin
One of Australia's finest metal bands also turns out to be one of the most underrated. During the 1980's in Australia, a big metal act touring from overseas was not a common occurrence and even when the big acts did come down under, every tour was very few and far between. Indeed, there was a hunger in Australia for local metal; and it was this hunger that brought about the formation of Mortal Sin in 1985. The band released their first album, Mayhemic Destruction, in 1986, and became known as the first Australian thrash band. Their sound was in the tradition of early Metallica, Slayer, Testament, and Overkill. The band continue releasing albums and touring to this day.
Carcass
While in their early days, Carcass originally focused on more of a grindcore meets death metal sound. They eventually evolved into more of a solid melodic death metal band, that developed and expanded a dedicated following. Fans would often compare Carcass to being "a death metal version of Iron Maiden". While not everyone agreed on this concept, all of the fans continue to agree to this day that through their evolution as a band, Carcass always stayed true to concept of the band and their music.
Napalm Death
Napalm Death are one of the founding bands of the death metal and grindcore scene and it is possible to argue they were the first band to create the grindcore sound and style. Despite being a band that has had perhaps the most line-up changes of any band in history (Napalm Death are a four-piece line-up. A tally of current and past members totals to 18), according to Wikipedia they are the seventh best-selling death metal band of all time. So they should be, even with all of the line-up changes, the band always continued to release good quality crushing death metal.
Morbid Angel
While Napalm Death hailed from England, on the other side of the Atlantic one of the first American bands to help develop the death metal scene state-side was Morbid Angel. Despite being plagued by continual line-up changes like Napalm Death, Morbid Angel continued to bring out some of the more darker and aggressive US-made death metal during the 1980's and into the 1990's, while also embracing some of the finer edged elements of thrash metal in their music as well. Their best known album is Covenant, which reached number 24 in the charts. Interestingly, each of their albums is named alphabetically (Altars of Madness, Blessed are the Sick, Covenant, etc.) as they are released. The band is due for a new release at some point during 2007, and will no doubt give it a title beginning with I.
Vader
Poland may not be known for its metal, but the country certainly gave metal heads probably one of the best-known European metal bands of all time. Uncompromising, aggressive, and unique, Vader brought a new light to the sound of death metal. While they were clearly inspired by many of the underground death bands from the early 1980's, their approach refuses to remain focused on traditional musical principals. Indeed, Vader have often sought to develop their own unique death sound; and continue to do so to this day.
Cannibal Corpse
Probably more well-known for offensive album covers and equally offensive (borderline disgusting) song titles, Cannibal Corpse was a band that would npt be forgotten after they came out. Based in the elements of death metal and black metal, the band developed a more extreme version of metal; with content that found its home in subject matter focusing on murder, abuse, cannibalism, and the glorification of mutilation. Not that you'd really know they were singing about it if the lyrics were not provided... In actual fact, it was their early albums that caused controversy in Australia, when certain albums by bands like Cannibal Corpse, Pungent Stench, and others were banned. This act saw Gestapo-like techniques of police officers raiding music stores and taking the CDs off the shelves and confiscating them. The efforts were ineffective: Cannibal Corpse have since become one of the best selling death metal bands of all time.
Motorhead
I would hazard a guess and state that it is most likely impossible for Motorhead to sell out. Lemmy came close when he had a cameo role in John Wayne Bobbitt's porno though. Motorhead have been a cornerstone in the metal scene for decades, and as historical influences are concerned, they left a hugely significant impact on the face of metal and with the bands that would follow them. While diversifying their lyrical topics and themes though, Motorhead still remained able to stay true to their fan base, and continue making influential metal today. Lemmy Kilmister and Motorhead have never changed their focus and beliefs, and will always be looked at as one of the more significant metal bands that never lost sight of what they were all about.
Testament
Like Overkill, Testament is another underrated metal band from the 1980's and 1990's. While they did record an album called The Legacy, which was a more polished version of the band, it still had a much harder edge and solidarity than other commercial efforts by bands like Metallica and Megadeth. Aside from this, the band, since their beginnings, have provided metal heads with some amazing and detailed compositions that was probably some of the most solid sounding metal for its time. Testament continues to this day, and is still able to kick it out intensely onstage as well.
Dio
He's been a part of some of the biggest metal bands ever. Artists such as Henry Rollins point to him as a significant musical influence. Ronnie James Dio has to be on this list because... well, he is Ronnie James Dio, damn it! Dio first broke into international stardom when he became vocalist for Rainbow, and eventually joined Black Sabbath following Ozzy Osbourne's departure. Eventually forming his own solo band, Dio has always had the same sound, the same attitude, and the same impact on metal. From his Holy Diver days with Dio, to his efforts in Black Sabbath and Rainbow, Ronnie James has always and will continue to show fans what metal is all about.
Death
Many have argued that the term "death metal" originally came from this band. Indeed, their debut album, Scream Bloody Gore, is considered by many to have been the model upon which future death metal bands would develop their art. Coupled with the death metal efforts by Napalm Death and other English bands, and along with the efforts of Death and Morbid Angel in the USA, the genre of death metal developed into something that would forever leave an impact on the sound and evolution of metal into the 1990's and the 2000's. The band came to an end when founding member Chuck Schuldiner sadly passed away from brain cancer but the band's legacy in the brutality of their music.
Meshuggah
Hailing from Sweden, Meshuggah are probably the most experimental metal band in the current scene. While their popularity has been a more recent development, the band has been around since 1987. Coupling their solid metal and progressive approach is their efforts to develop complex drum patterns, and odd time signatures has proven to have given the band an extremely unique, identifiable, and defining sound. Perhaps one of their more well-known releases was an EP called I. The EP was one song and plays in total for about 20 minutes. The recording was an epic result and is often pointed to by fans as their one of many of their defining moments.
Faith No More
The funk metal scene came and went as the hair metal scene died and grunge took over; and although Faith No More embraced that sound in their early days, they were a band determined to evolve and not succumb to far too much commercialism. Their album The Real Thing and accompanying single Epic were their breakthrough moments into international success. However, instead of following up this album with further commercialism, to their credit, Faith No More's next album, Angel Dust, was heavier, weirder, and very experimental. While the rest of the releases never touched the success of The Real Thing in the USA, each successive release was more popular than the last in countries like Australia. After the band's demise, frontman Mike Patton continued recording, involving himself in unique projects such as Fantomas.
Sacred Reich
Another underrated band that began in the 1980's, Sacred Reich was often lumped into the thrash metal genre but something about them stood out from the others. While they did not seem to fall into the traditional template for thrash and speed metal, their sound was often more concerned with solidarity and impact; and their lyrical concepts more politically inclined - written with much quality and thought that many could argue they rivaled the political lyrics of Dave Mustaine. The band broke up in 2000, but will be reuniting this year for some shows in Europe.
Dream Theater
Dream Theater's progressive sound often has to be heard (and seen) to be believed. Check out some of their DVDs and you can see how intricate and detailed their compositions are - this band obviously spends a lot of time in song development. Add in there that this band was bad-ass enough to perform Metallica's Master of Puppets album live in its entirety; and their phenomenal recent performance with an orchestra onstage, and you will find that it's probably impractical for a band like Dream Theater to even consider selling out. They are always on a constant evolution, but remain true to their vision of being one of the best (if not the best) progressive metal bands out there.
Demon Hunter
Think Killswitch Engage mixed with Pantera mixed with Machine Head, and you might get a tiny microscopic idea of what Demon Hunter sound like. Perhaps one of the more unique metal acts to surface out of recent years, Demon Hunter deliver an aggressive metallic sound, infused with some melody here and there. To understand Demon Hunter, you have to listen to them to find out what they are about... Their most recent release was The Triptych, which has been pointed to by their fans as perhaps their best release yet.
Living Sacrifice
Before drummer Rocky Gray joined Evanescence (and before they ripped off the unique sound and style of Italy's Lacuna Coil), he was part of a band called Living Sacrifice. Coupling elements of death and thrash metal with the rising sounds of metalcore, Living Sacrifice, like Demon Hunter, made their beginnings as a Christian metal band. The band released 6 albums from 1991 to 2002, all of which remained consistent to their sound and style. In the metal scene, those who were fans of the band sorely miss their presence on the circuit...
A common criticism of metal bands, such as 
Strapping Young Lad / Devin Townsend
Henry Rollins
Slayer
AC / DC
Immortal
Iron Maiden
Pantera
Overkill
Lamb of God
Mortal Sin
Carcass
Napalm Death
Morbid Angel
Vader
Cannibal Corpse
Motorhead
Testament
Dio
Death
Meshuggah
Faith No More
Sacred Reich
Dream Theater
Demon Hunter
Living Sacrifice
