At a glance:  GEISS

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Watch the sound waves ripple and swirl as you fly through a dreamlike realm of sight and sound. Download the Winamp plug-in version to run it using Winamp and mp3's, or get the stand-alone screensaver, which can react to the microphone, a CD, or your computer's stereo mix.

This was my first Winamp music visualization plug-in, released in 1998, and quickly became a popular favorite; to date, it's been downloaded over 3.5 million times from alone. (Okay... to be fair, back in the early 2000's, that was a more impressive number. :)

Geiss was updated in 2007 to support arbitrarily high resolutions, so it now runs nicely even at HD resolutions such as 1920x1080.

You can also check out Geiss 2, which I wrote in 2003. For some reason, it doesn't have the beauty of the original, but it does offer more features - animated transitions, desktop mode, multiple monitors, and automatic core loop assembly optimization (for maximal speed on any machine).