I love this video - it brings together two of my favourite things - homebrew hacked robots and music/rhythm experimentation. It uses the Picaxe controller board, caterpillar tracks, and a sonar distance sensor.
The Yellow Drum Machine robot taps on objects, samples the sounds, and assembles beats with them. An interesting way to use a robot, and quite endearing.
For more information on the robot and how it was made - see this: Yellow Drum Machine | Make. It includes full building instructions.