Windows, which I run, and the two most popular there are Centrafuse from FluxMedia and RideRunner from NISU Automotive.
Centrafuse is about to release version 3.0, which is a complete re-write. I have the latest Release Candidate running crash-free on an EeePC netbook; it looks terrific, and is an easy install. The Centrafuse thread is here.
RideRunner released its latest build early last month. The RR thread is here.
I've tried both, and chose Centrafuse for my system, but that's just my preference. You have lots of options with either package.
You'll have different options with Mac or Linux, but I can't offer information about them. The MP3Car Front Ends thread is here.