Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Visualify - Visualize your top artists on Spotify
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Spotify Web Visualizer - Beautiful full screen visuals for Spotify web player
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Kaleidosync - Kaleidosync is an all-in-one Spotify music visualizer that is all set to deliver a top-notch music experience with real-time support.