I have one of the 80 gig ones for those long drives at work. But I listen to a lot of metal and classic rock and stuff like that. Some of the main groups I like listening to are:

Black Sabbath
Led Zeppelin
Bob Seger
BTO
CCR
Fleetwood Mac
John Hiatt
Lynyrd Skynyrd
Metallica
Megadeth
Mudvayne
Three Inches of Blood
Nirvana
Pink Floyd
Rolling Stones
Steppenwolf
The Doors
Tragically Hip
The Who
Tool

I have lots more but that's the main stuff.