Buzz is one of few programs mentioned here that have created a real user following (the others being Generator, Reality and to a lesser extent VAZ). It is described on the BuzzTrack site as 'a portable, free Pro-Tools system for the rest of us.' Well I wouldn't go so far, but it is a very interesting program.
Buzz is basically a tracker, but is far advanced to any I have seen before. This means it does not support MIDI note input or use with an external sequencer - it does support MIDI output however and MIDI controller input.
It is based around a modular concept. You have generators (plugins that create sound), which feed into effects modules (plugins that change sound) which feed to the master output module. VST plugins can also be used as effects. All this is editable from within a graphical modular diagram (see screenshot below). There are many people creating generators and effects modules. By double clicking on a module, the parameters appear and they can be changed in realtime during song playback. Sequences are created against generators using a grid type view and are then triggered off from a master grid which has a column for each generator.
The output is incredible compared to any tracker-type software I have heard before. Many of the generators available are up to the quality of other soft synths available. It will be interesting to see how it will develop in the future. I can imagine a single environment into which the output of a sequencer (both MIDI and audio) could be piped and which would provide a unified host for soft synths and audio processing, getting nearer the idea of a studio within a PC. The possibilities are endless. Buzz is currently freeware.