I've got 4 play lists that I like to listen to:
(1) "Drive Hard" -- all high energy techno with a thumping beat
(2) "Mellow Drive" -- slow folksy stuff like Nora Jones, Jack Johnson, Coldplay, Dido and stuff like that.
(3) "Sing-along" -- just a bunch of songs that I know the words to and like to sing (or scream) along with... including a bunch of 80's stuff, Erasure, Kelly Clarkson, and showtunes (Avenue Q, Rent, etc). Yes, this is my uber-gay playlist... and I LOVE it!
(4) Podcasts -- usually listen to Al Frankin, or some tech-geek stuff.