Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Figure - Propellerhead creates world-class software products and services that inspire music makers and provide the foundation for a worldwide creative musical community.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
SoundGrail - SoundGrail Music App designed for pianists, guitarists, and DJs.
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
keezy - A colorful soundboard. Play with music.