Tool, duh.

Fugazi (Straight Edge band....but OH MY GOD is that shit f-u-c-k-e-d up)

Aphex Twin

NiN

Strangely enough, Against Me! is great to listen to while tripping.

DJ Tiesto

Beastie Boys have some pretty trippy tracks, actually.

Prodigy, of course.

Postal Service if you're looking for a chill, mellow, trip.

Bumblebeez 81.

CCR

blah, n;sj, halb.