sells John Dunn's midi software. Many years ago he made
a very interesting freeware MAX precursor called Music Box. Apparently
you can't get it from him any more, but I have the source code somewhere,
so I should link to it.
Here's a system for composing MIDI sequences, describe in a paper by Alin Dorin, that uses Boolean networks (something like cellular automata) to generate rhythmic structure. Has a colorful user interface, and runs on Macintosh.
Boomer Bag is a web repository of rhythmic and harmonic fragments. You can listen to the contributions, rate them and submit your own.