Voicemeeter - Audio Processing and Audio Programming by V.Burel
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
Soundflower - Soundflower is a Mac OS X system extension that allows applications to pass audio to other...
AudioBus - With Audiobus, the revolutionary new inter-app audio routing system, you can connect your...