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