There's actually a Yahoo category dedicated to in-car mp3 players:
Check also http://www.mp3car.com/
I'm not automotively-MP3 minded, but at home I've been using
X11Amp - http://www.x11amp.bz.nu/index2.html
It's a pretty smooth looking version of the windows winamp
player, and it comes in an amazingly small 118,820 byte rpm file!
How do they get so much stuff in a package so small??
Anyway, the most fun I've had with MP3 was just a few days ago -
I downloaded Van Halen "Jump!", and spent an hour or so with my
kids, blasting the volume, and all of us "jumping"
along with D L Roth :-).
Fun. Brings the value of the Internet home to you, when kids too
young to type, or even to comprehend the contents of a video screen,
can enjoy the benefits of downloaded music.