MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Amarok - The Amarok team is proud to announce the immediate availability of the result of 2 years of hard work, the beta release of Amarok 2. 9. This release is marked by improvements, corrections and polishing of nearly all Amarok features.
VJmachine - Music Visualizer - Make a VJ show like a PRO. Music visualizer for a unique tight symbiosis of sound and video.
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Quod Libet - A extensible Music player with Podcast support, web radio, playlists and many more.