MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
VLC Media Player - VLC is a free and open source cross-platform multimedia player and framework.
VJmachine - Music Visualizer - Make a VJ show like a PRO. Music visualizer for a unique tight symbiosis of sound and video.
foobar2000 - An advanced freeware audio player for the Windows platform.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Winamp - Winamp is a media player that allows users to play multiple-file formats, arrange media in a varied file-management system, and play unique media with AAC encoding.