http://seaquence.org/
It's more an experiment in sound then anything else, but it's a neat little thing to doodle with.
Basically you create little fish guys that correspond to a specific sequence of notes. After getting them to sound how you'd like you let them roam the water, a circular region with a constant beat pulsing defined in the center that they are drawn to (upon getting pulsed, they play their sequence) - having only a few able to fit at a time a time they get drawn in and out of it's perimeter based upon who's in there already the "song" randomly changes and evolves over time.
It's not really anything you'd do much serious in, but it is a neat little thing to doodle in and experiment with other less conventional methods of composition.