Update: I started out using the Wii Nunchucks Accelerometer for control, but I quickly tired of it and wanted to make my own controller.
I've only been into arduino for about 6 months now, and this was kind of a bigger build for me, but it has been one of my goals since I first got my Uno. I built an RC car using an Uno, a breadboard arduino, two RFM12B wireless transceivers, and a Wii Nunchuck controller.
The transmitter side currently uses the Uno (will soon be swapped out for a breadboard arduino), the Nunchuck's accellerometer (library from todbot.com)for speed and direction control, and an RFM12B (using library from jeelabs).
The receiver side uses a breadboard arduino, and the other RFM12B. From there, a dc motor is driven from a TA8080K motor driver (forward and reverse, but no speed control). This will be swapped out for a motor controller board with PWM that's on the way from sparkfun. Steering is done with a servo, and the who thing is currently powered by a 9v into a 7805 5v regulator (will be switched to a AA pack in the future). I currently have it all mounted to a piece of plywood just to get it all working, but I have plans to move it to a real chassis in the future.
Here is a video of it running: