The 6 CDs in my Prius right now:

Little River Band's Greatest Hits
Kraftwerk - The Mix
Pierce Pettis - Making Light of It {since most haven't heard of him, I'll provide genre: folk}
Depeche Mode - Catching Up With Depeche Mode
Beastie Boys - Hello Nasty
Big Audio Dynamite - Planet B.A.D.

Now, having provided that list, I have to admit that 95% of the time, I listen to a local news/traffic/weather station. 95% of my driving is commuting, and the traffic reports are essential. As I near my destination, I start ignoring it, but I usually forget to switch over to music.

If the radio isn't tuned to new/traffic/weather, then I bounce between a country station, an "urban music" station (rap), a "classic" rock station, and an alternative rock station. Whichever one I come to first that isn't playing a commercial or talking.