MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Bullet - Share captioned video snippets of podcasts from any app
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Box2D - A 2D Physics Engine for Games
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Matali Physics - Matali Physics is an advanced, multi-platform, high-performance 3d physics engine intended for games, virtual reality and physics-based simulations.