A good idea - though one that I'll shamefully plug as something that's been in the back of my head for a while, but never matriculated into a product - is the VMOD car-mountable box that allows you to easily transfer music, video, photos and other media to your car via a Wi-Fi connection, giving you bragging rights for anyone who thinks their stereo deck is cooler than yours.
From the inside, it's simply a pre-built compact Windows XP PC with a slick interface via StreetDeck. With the built-in WiFi, you simply set it up to sync certain folders on your home computer and it will automatically copy over the correct files whenever it sees a difference in file structure. Also, by allowing the ability to also add on GPS and satellite radio capabilities, the VMOD allows even further expansion on top of the already robust feature set. And for those going off-road, the shock-resistant 40GB hard drive should prove useful for bumpy roads.
The complete VMOD system be available for about $1000.