Panasonic CQ-RX400U Car Stereo
I usually don't bother too much with upgrading OEM equipment, but the last owner of my car made that decision for me once already. But, the Sony receiver that they put in her has a freak electrical problem. So, since I have to buy a new one, I decided that I wanted one with front auxillary support. Auxillary support makes using a mp3 player that much easier - headphone port to auxillary port and you're done! But, this one also has USB support. You can use any thumb or flash drive as an media player; just plug it into the USB port in the face.
For people with greater music demands, here's a 10-disc changer from Kenwood. It plays CD-Rs and CD-RWs and allows for 100-disc naming with a compatible Kenwood receiver.
This Sony CD receiver conveniently plays MP3s and WMAs, so instead of being confined to 20 tracks, you can shove in a data disc with 100+ tunes on it. It has a USB input on the front panel for connecting other digital devices.