Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
VLC Media Player - VLC is a free and open source cross-platform multimedia player and framework.
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
foobar2000 - An advanced freeware audio player for the Windows platform.
Plane9 - Plane9 is a revolutionary 3d visualizer where you never have to settle for just one view ever again.
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.