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.
Example: To send audio from iTunes to Max/MSP, open the System Preferences and select Soundflower (2ch) as the device for sound output inside the Sound panel. Then, inside Max/MSP's DSP Status Window, select Core Soundflower (2ch) as the input device. All output of iTunes should now be sent to Max/MSP's adc~ object.