Reaper Audio Production - A pretty cool little program, little being the key word. This little guy weighs in at 2.4 MB! Alot like ACID with less features.

Psycle - A tracker interface based music creation studio with VST, VSTi, and MIDI support.

Music Studio Producer - A studio application that includes a piano roll, sequencer, mixer and more.

Mixxx DJ Software - An open source DJ program providing everything you need to make live mixes.

Delta SP Music Software - Delta SP

Buzz Machine - A popular modular audio sythesizer, utilizing "machines" and graphical wiring to allow flexible audio routing.