Speck ToughSkin - I bought this $30 case to protect my $300 iPod photo 20GB. The rubber bumpers protects it from shocks and drops as well as scratches. It makes the iPod bulkier but well worth it. I use the belt clip on it all the time. A lot of the other cases/skins will protect against scratches but they don't really help when it comes to drops.
OtterBox for iPod - If you truly want protection from the elements, this case is pretty tough to beat. It's waterproofed up to 10 feet and completely seals the iPod from dirt, sand, and water. The click wheel is still accessible and has a waterproof earphone jack. Waterproof headphones not included.
Newer Technology RoadTrip! charger/transmitter - I'm too cheap and/or too lazy to install a car stereo with auxilary input jacks. So I use this car charger that doubles as an FM transmitter. It's FM transmission is surprisingly stronger than alot of the other FM transmitters I've tried.
Car Stereo with auxilary input jacks - If you truly want great sound, get this for your car. Connecting an iPod to the auxilary jack will sound miles better than any FM transmitter.
AirClick for iPod - I connect this remote control when I'm in my car or in my bedroom. It's a nice simple controller. It has audio up/down, previous/next track, play/pause, and power on/off. I've heard good things about the Targus remote control but the ultimate would probably be the Ten Technology NaviPro EX Wireless iPod Remote Control. It's a much bigger remote but allows you to switch between songs, albums, and playlists.