I play mostly funk/groove/jam experimental stuff, but everything from folk to dub to straight-up rock gets thrown in the mix.
Like John, I've got a couple hundred hours of sound on my computer which I set to random and I'm usually pleased with the results. In heavy rotation are Frank Zappa (Tom Fowler & Arthur Barrow are great), Ry Cooder, King Crimson, Hot Tuna, Nick Drake, John Cale, Ween, Stanley, Jaco, Captain Beefheart, Steely Dan, Jeff Beck, Les Claypool (in varying lineups), Leo Kottke, Al DiMeola, Paco De Lucia, Jean-Luc Ponty, and so on, and so on...
Recent additions that I've been listening to in the car are Roxy Music, ELP, Pere Ubu... Interesting stuff! I have found that generally speaking, the earlier in the artist's recording career, the more I like the music. Ry Cooder's early stuff is some of the best, IMHO.
Adam