MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
Youtube-DJ - Mix YouTube songs together
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
Pacemaker - Create and share mixtapes with friends
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
djay Pro for iPad - djay Pro provides a complete toolkit for performing DJs.