Batuhan Bozkurt has created a simple cellular automata music generator that runs in Flash in a browser window: Otomata. You don’t need to understand cellular automata to play with it, and it generates fascinating, lovely music (he chose an interesting scale, and shapes the tones very nicely).
Bozkurt has a lot of other interesting computer generated music, based on tools he writes. He wrote a very interesting effort (with code and audio examples) to recreate the THX audio logo (which you hear at the beginning of any THX movie). Reminds me of the famous crescendo in the Beatles’ “Day in the Life”.