It's a damn shame I'm not going through my heavy metal phase right now, it sounds like there's a ton of great bands out there. Then again, for the sake of my credit card, it's probably for the better I'm not listening to a lot of metal.
Contribution:
Remember that show I mentioned earlier, "The Rolling Stones Rock and Roll Circus"? Well, although The Who don't really fit the bill of underrepresented bands, I'd say this song probably does. To my knowledge, it's not on any of their regular albums, but it damn well should be (With my luck, it probably is and I just don't know about it. Either way, it's a rare live version that completely kicks ass):
A Quick One While He's Away [http://www.youtube.com/watch?v=GjyZ6B7mC-w]
Fun bits of trivia -
One of the purported (though unverified and unlikely) reasons that this particular show was not released for so long is that some claim the Rolling Stones were jealous at being upstaged by the Who, who had just gotten off a world tour and were in top form. The Stones, on the other hand, had not and are quite obviously high for their act.
At the time of this recording, Keith Moon was still their drummer (He would later die of an overdose (Don't do drugs, kids)) and is the one playing drums in this video. He was, according to popular rumor, the inspiration for the Muppet "Animal"
For something more in line with the thread and underrepresented bands:
Hard Rock
Black Tide - Shockwave [http://www.youtube.com/watch?v=rU9DCB2qL1Q]
Hip-Hop/DJ/I don't know what to call it
Cut Chemist - The Garden [http://www.youtube.com/watch?v=gHhlaspXVgo]