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
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
MIDITrail - "MIDITrail" is a MIDI player which provides 3D visualization of MIDI datasets.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Piano Connect - Piano Connect is one of the most popular piano learning apps that helps you play piano on your smartphone anywhere at any time.