Cheap ($15) ($14) Robot Swarm developed at Harvard
I initially overcharged my readers by $1, corrected.
Just a cool link to the Kilobot project: an inexpensive platform for exploring cooperative robot swarms. The robots themselves are rather like Bristlebots: they use a pair of vibration motors which bounce the robot on rigid legs, allowing them to turn and move over smooth surfaces. They also employ a rather clever communication technique using infrared leds which are carried underneath. The light bounces off the tabletop and can reach nearby robots, or an overhead master beacon controller can broadcast programs and data to all robots simultaneously.
More information and videos of the Self-organizing Systems Research Group