Jack OS X for Mac

From Defective Records Software:

Jack Tools is a Mac OS X implementation of the Jack Audio Connection Kit (Jack). Jack is a low-latency audio server, written originally for the GNU/Linux operating system. It connects any number of different applications to a single hardware audio device; it also allows applications to send and receive audio to and from each other. Jack is different from other audio server efforts in that it has been designed from the ground up to be suitable for professional audio work. It focuses on two key areas: synchronous execution of all clients, and low latency operation.

Jack Tools installs everything necessary to take full advantage of Jack on OS X. This includes:

• The Jack server
  The infrastructure you need in place to use Jack.
• The Jack Audio Server (JAS)
  JAS is a CoreAudio "user space" driver that allows any OS X CoreAudio application to become a Jack client.
• The Jack audio plugins
  Both AU and VST "Jack-aware" audio plugins are provided, which further expand the limitless audio routing possibilities when using Jack. Both VST formats (Mach-O and CFM) are provided.
• The JackPilot application
  JackPilot offers an easy to use GUI interface that allows you to control the Jack server, and manage the audio connections between applications and/or plugins.

Best of all - it's free!

Jack Tools - connecting audio from any OS X application to any OS X application!