Solarbotics has enhanced Herbie a bit with functional whisker and tail sensors, so it doesn't get stuck in corners while it chases around. Herbie the Mousebot is a 9-volt battery-powered robot that loves to chase flash light beams. If there are several Herbies in the same area, they can be configured to chase each other! (I did not install this feature yet, as it uses additional power and can shorten the battery life a bit. It basically involves installing and infra-red LED on the back of Herbie as a tail-light, which another Herbie can chase). Solarbotics says, "These little robots are so quick, you have to run to keep up to them!" They are not kidding!
Herbie documentation can be found here.
Note - much of the above is taken from the Solarbotics site at the link above.