MilkDrop - 3D audio visualization plugin, compatible with WinAmp plugin API
WaveShop - Open source audio editor
Magic Music Visuals - Fully-customizable and highly responsive visuals for live and recorded audio & MIDI.
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Plane9 - Plane9 is a revolutionary 3d visualizer where you never have to settle for just one view ever again.
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.