Soundflower is a MacOSX* system extension designed for inter-app audio routing (it allows apps to pass audio to other apps).
The app has been designed to be very easy to use; it operates as an audio device, thereby allowing any audio app to send and receive audio with no other support required.
For example; if you need an app to interact with another via the soundcard, simply select Soundflower as the output device in the first app and the same as the input device in the second app.
Soundflower is an effective, free audio routing solution that has some cool features such as the ability to allow different apps to access the soundcard simultaneously. It also supports 2 and 16 channel audio. The downside is that some may find Soundflower tricky to configure at first.
*OS X 10.5.8+ is required to run Soundflower.