Sampulator - Make (and record) beats on your keyboard
Music For Makers - Licensing good music has never been this easy.
Tinkersynth - Create and purchase unique generative art
TunePocket - Download 11,000+ royalty free music tracks and sound FX. Buy once, use forever. New music added daily. Save over 50% compared to most popular similar services. Incredible value!
Splice Beat Maker - Make and share beats in your browser
Incompetech - Royalty Free and Creative Commons Music Archive