The Mars Volta isn't exactly my favorite band, but they're WAY the fuck up there... Though, I'm used to be the only one who likes them. They're basically a prog-rock band firmly ensconced in deep left-field, I guess. I tend to have a bit of trouble describing them, just because the closest contemporary I've ever found for them is Coheed and Cambria, which I hate. Coheed is a fine band in their own right, but The Mars Volta pretty much outshines them on their progressivist musical theory and post-hardcore tendencies (actually, this is one of the bands that formed after At The Drive-In broke up. Same singer and guitarist, just producing more worthwhile material). I think the best way I've found to describe them is "Pink Floyd for meth-heads."
Anyway, have some examples (I would like to take this time to note that because of YouTube's dumbass policy in regards to video length, I opted out of not uploading a few songs because I didn't want to have split videos for one song. This is why there's about a minute missing from the 'Meccamputechture' upload. Sorry that YouTube sucks):
Eriatarka
Son et Lumiere + Inertiatic ESP
Meccamputechture
Vermicide
Goliath
Televators
Cavalettas
Since We've Been Wrong
I also feel obliged to tell you that they won a Grammy for this song, 'Wax Simulacra', which I find to be a piece of shit, but whatever.