Synchronous Audio Router - Routes Windows application audio to and from your digital audio workstation software.
Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
AudioBus - With Audiobus, the revolutionary new inter-app audio routing system, you can connect your...
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
SoundBunny - SoundBunny allows you to control the volume level for all open applications on your Mac.