Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
Soundplant - Play sound files with the computer keyboard
MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
CasterSoundboard - A soundboard for hot-keying and playing back sounds. (For podcasting)
projectM - projectM is an LGPL'ed reimplementation of Milkdrop under OpenGL.
JN Soundboard - An open-source hotkey soundboard program