MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
VJmachine - Music Visualizer - Make a VJ show like a PRO. Music visualizer for a unique tight symbiosis of sound and video.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Morphyre - Animated, 3D Music Visualization with thousands of unique scenes.
Kauna - Kauna is a music visualizer. Kauna renders any sound reproduction in system.