Fast line-following robots
This is really inspiring.  I played around with converting an RC car to self-driving with an arduino but it was very simplistic. It used an ultrasonic sensor (Parallax "Ping)))" sensor) to try to keep a specific range of distance from a wall to its left.  It wasn't aware of its angle so it had a positive feedback loop in its reactions which caused it to oscillate more and more out of control.  Just knowing there are people are there playing around with this for fun makes me want to get back into it.