MilkDrop 2
A flexible, GPU-driven music visualization engine.
MilkDrop is a music visualizer - originally a "plug-in" to the Winamp music player, and now available as OSS.
As you listen your music in Winamp, MilkDrop 2 takes you flying through visualizations of the soundwaves you're hearing, and uses beat detection to trigger myriad psychedelic effects, creating a rich visual journey through sound. MilkDrop 2 can also be driven by a live audio feed (microphone or line-in) - see the documentation for details.