Radium - Radium is a music editor with a new type of interface.
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Keezy Drummer - Make a beat in seconds
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
BlokDust - Join blocks together to build sounds with this web-based music making app.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.