projectM

Over 40,000 user contributed Music Visualizations - See what you hear with projectM - The most advanced open-source music visualizer.
Play music with other apps, then visualize that music with projectM!
projectM is an open-source project that reimplements the esteemed Winamp Milkdrop by Geiss in a more modern, cross-platform reusable library.

Its purpose in life is to read in audio input and produce mesmerizing visuals, detecting tempo, and rendering advanced equations into a limitless array of user-contributed visualizations.

Observe the sounds around you with your eyes.
Captures windows system audio and renders visuals to the screen.