EricDuino’s Line Follower Organ plays notes based on the curves it follows. (Translation via Google):

This is the hijacking of a line follower robot capable of interpreting its track as a partition. The music stand of the instrument is a continuous curved line traveled by the instrument. Physical variations of the linearity of the curve and the induced behavior to the object affect the sound quality is always approaching the target but never identical. Any closed curve drawn in black on a white sheet represents a partition loop that will be played approximately.

[via Arduino]