Best money I've ever spent on audio equipment: iPod and a Belkin FM modulator.
A few weeks ago I picked up a 20gb iPod because the price was right (was on sale @ Circuit City). I also got a little FM transmitter with digital tuner that plugs into the headphone jack (which turns the iPod into a tiny radio-station). I've loaded the thing up with all my music and created a bunch of playlists. Now I listen to it when I'm mowing the lawn or working in the garage (most of the work I do in the garage requires I wear hearing protection so I can't listen to a regular stereo).
The best part is that when I get in the car I just plug the little FM modulator into the iPod and hit preset-1 on the car stero and I'm listening to the iPod on the car stereo. It works wonderfully - sounds great and is as small and portable as a deck of cards. I've saved a pre-set FM station in both cars so I no matter which car I'm in I just pull the iPod out of my pocket, select a playlist, and crank up the car stereo to hear my own personal radio station. No commercials. All music all the time
This would allow you to use any head-unit you want as long as it has a decent FM tuner.
-Ben M.