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.

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).