MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Visual Beat - Interactive music video
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Google Music Lab - Interactive experiments that use the Web Audio API
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
SoundViz - Turn music and sounds into beautiful art