I took an off-road R/C car chassis, and built a custom control system to navigate along a chain of GPS waypoints.
This project was an independent study in my senior year of high school. The truck navigated to a series of GPS waypoints, using a GPS, magnetometer, and custom code. I etched the controller PCB, which held two serial-linked AVR microcontrollers to handle the navigation and sensor interfacing.
During testing, bugs in the code sometimes caused the vehicle to randomly bolt away at around 30 mph. After it ran under a truck and hit a lot of curbs, I eventually put together a remote kill switch, using a long-range XBee tranceiver.
This vehicle was originally intended to compete in the SparkFun Autonomous Vehicle Cometition, but I wasn't able to travel to Colorado in the end.