This is a simple robot designed by my daughter, and featured in TheShed magazine in Aug/Sept 2010.
HaloBOT article
(Click on the images for larger views)
It uses a Picaxe 18 and L293D to control a Tamiya twin motor gearbox.
The bumper is designed to be non marking, while allowing left, right and straight
ahead to be detected with only two micro-switches.