Here is a bot I made from some stuff I had lying around. It is kind of a typical RC car platform with an Arduino Uno. I had picked up a few Unos and Seeed Motor Shields at some Radio Shack clearance sales.
The "eyes" are a Parallax Ping ultrasound sensor from the same Radio Shack sale.
Right now it just backs up when it detects something within 5 inches of the ultrasound device.