I have to hear "Under the Milky Way Tonight" by The Church every year, after I pass through Cedarville, sometime before midnight. (I don't know why, man--I don't make the rules.
) I add stuff to a Burn playlist all year and then strip it down and customize it depending on who I'm riding with. I like to surprise/entertain/delight my co-pilot at least half the time 1) because it's fun, and 2) so that they won't get too cross during my flights of fancy and fling my iPod into the Klamath river. If I drive myself this year, the music will be slightly mellower than it would be otherwise so that I don't speed (unlikely to happen) or drive off the road (of medium likelihood). If I have company, the gamut will be run.
Right now I have 152 songs. I have a tendency to grab several per album. Prominent right now:
Anthology Through the Years - Tom Petty & the Heartbreakers
Absolute Garbage - Garbage
Back to Black - Amy Winehouse
Best of Bill Withers
Blue Lines - Massive Attack
Carnivalé (soundtrack) - Jeff Beal
The Essential Clash
The Brit Box: UK Indie, Shoegaze & Britpop Gems
The British Beat: Best of the 60's
Beggars Banquet - Rolling Stones
Contraband - Velvet Revolver
I Told You I Was Freaky - Flight of the Conchords
Icon 2 - Rob Zombie
The Fame - Lady Gaga
Soundtracks - Kill Bill 1&2, Pulp Fiction, Pirate Radio, The Sopranos, Swingers, Take the Lead, True Blood, Donnie Darko
Neon Bible - Arcade Fire
Midival Times - Midival Punditz
Purple - Stone Temple Pilots
And there will be some of "The Lonely Island" on there. Dance, trance, industrial, funk, hip hop, goth, pop; a little of everything.
I like to use the online hold at my local library to pick up CDs and drop 'em to iTunes. If I can't think of what I want, I drop into the library in person and hit the CD soundtracks first.