Soundflower is a OS X system extension that allows applications to pass audio to other applications. Soundflower is easy to use, it simply presents itself as an audio device, allowing any audio application to send and receive audio with no other support needed. Soundflower is free, open-source, and runs on Mac Intel and PPC computers.

Note: This is only the Soundflower kernel extension. The SoundflowerBed app is retired. A new replacement is in the works. In the meantime, you can take your chances with an older version of Soundflowerbed, but you can do a lot with a Multi-Output Aggregate Device.