I have two 1GB CF cards full of MP3's I always keep in the car.

Bach's Toccata and Fugues for organ
Martha Argerich's recording of Chopin
Aaron Copland's Appalachian Spring, Rodeo, Billy the Kid, and Fanfare
George Gershwin's Rhapsody in Blue & An American In Paris
Howard Shore's Lord of the Rings soundtracks (all three films) - can't wait for The Complete Recordings for TTT and ROTK
Mozart's Clarinet Concerto
Strauss waltzes conducted by Bohm and Von Karajan
Some ballet suites by Tchaikovsky
Pachelbel's Canon and Concerto for Trumpet

That's only a small sampling but I've nicely filled out a classical collection. That's not all the music I like. I have a bucketload of soundtracks (mostly John Williams and Elmer Bernstein) that I should put in the car. Lately I've been listening to Beastie Boys and Beck on my PC and plan on acquiring some old school rap and some alternative stuff.