I know Elysium's already got a very nice thread on free music from unsigned bands, but I thought it would be fun to make a thread specifically for the crazy avant/progressive/metal/rock drifting around out there on the intarwebs. Keep your crappy emo/techno/garage rock out of this thread, this is for strictly for stuff that is aggressively mind-bending, musically challenging, and occasionally borderline unlistenable! 
I'll start out my plugging some friends of mine:
Canvas Solaris
Instrumental experimental metal from this Georgia 3-piece. Tech metal assaults contrast with splashes of world rhythms, electric piano and synth, and acoustic guitar. In addition to their myspace tunes, a track from their upcoming album can be downloaded from their label here.
Another instrumental act:
Blotted Science
Crazy shred-tacular prog/tech metal from Spastic Ink guitar virtuoso Ron Jarzombek, featuring the bass player of Cannibal Corpse (really!) and the drummer of Behold...The Arctopus. The album's not out yet, but I predict that it will totally slay.
And speaking of Behold...The Arctopus...
More absolutely insane instumental technical metal courtesy of this Brooklyn trio. These guys are so over the top it's hilarious. And their songs are short, so if you don't like them they're over quickly!
And one more just for variety's sake:
Sleepytime Gorilla Museum
Non-instrumental, so avant-garde-it-hurts progressive rock. You may recognize singer/violinist/guitarist Carla Kihlstedt as the violinist from Mr. Bungle's brilliant California, and that will give you an inkling of what to expect from this band/traveling sideshow. Start your listening with the brilliant Sleep is Wrong and go from there.
That should be enough to hurt your brain for now. 
Pages