And what is Cthugha (pronounced "Kah-Thoo-Gah")? It's a free program available for PC's (running DOS, Windows, or Linux) and Macintosh's which takes sound input from a CD, stereo, or microphone and displays colorful animated graphics synced to the sound. Sort of like an oscilloscope, but on LSD.
Cthugha has become a very popular form of visual entertainment, useful for parties, concerts, raves, and other events as well as just vegging out to mesmerizing, hynotizing displays... More Info »
Editors Note: That official type description out of the way let me say this:
seen many programs that "respond to the music", I always come back to this program.
music. Changes in instrument, tempo, volume are all available visually. Cthugha allows you to experience the music in a whole new way that complements the aurreal experience perfectly.