An incredibly nifty realtime music composition program, written by crazy people from Finland. It runs on Windows, sadly, and was somewhat forward looking.
What makes Buzz more powerful than its bretheren is that is is a framework for connecting so called 'Machines' together, which can be written by third party developers. The Machines are capable of creating note information, like a sequencer, or or creating waveforms, or additionally filtering the noise in a variety of ways.
All these machines are tweakable, as part of the program that you create for buzz. Many Buzz songs are composed in a tracker-like interface, but this is not required, you could of course write your own input machine.
starting to get it?