MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Apophysis - Apophysis is a fractal flame editor and renderer for Microsoft Windows.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Xaos - XaoS is an interactive fractal zoomer.
VJmachine - Music Visualizer - Make a VJ show like a PRO. Music visualizer for a unique tight symbiosis of sound and video.
Ultra Fractal - Ultra Fractal is a fractal generation and rendering software application.