Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
MIDI Visualizer - Small cross-platform MIDI visualizer written in C++/OpenGL
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
MIDITrail - "MIDITrail" is a MIDI player which provides 3D visualization of MIDI datasets.
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Piano Visualizer - Bring your piano music to life with this application.