Splice Beat Maker - Make and share beats in your browser
WaveShop - Open source audio editor
Ramsophone - A generative art/music machine. (Be sure to refresh!)
TwistedWave - TwistedWave is one of the unique audio editors that allow you to edit your music file online without any installation.
Google Music Lab - Interactive experiments that use the Web Audio API
Audacity - Audacity is a free and open-source audio production software suite that includes a surprising array of editing tools and recording systems.