Visualify - Visualize your top artists on Spotify
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Synesthesia - Synesthesia is a Spotify music visualizer that comes with the advanced support to listen to thousands of music tracks with the visuals side by side.
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.