SuperColliderAU SC's server can become an Audio Unit
SuperColliderAU is a project by Gerard Roma that embeds a SuperCollider Server as an "effect" Audio Unit, so that it can be directly used to generate/process audio within Mac OSX software such as Logic or Ableton Live.
Installing:
Copy the bundle "SuperColliderAU.component" into /Library/Audio/Plug-ins/Components
Quick Start:
Start up some piece of software that can use Audio Units. If you don't have Logic or whatever, you can use the free "AU Lab" program provided by Apple as part of their Developer Tools. Set it up so that you have some source sound (e.g. an audio input) and then add an "effect" unit of type SuperColliderAU. A panel will appear telling you which port the server is listening to for OSC messages.
Now you can run this code from within the SuperCollider language to talk directly with the embedded server:
u = Server(\scau, NetAddr("127.0.0.1", 9989)); // Change the port from 9989 to the right one for you
u.serverRunning = true;
u.ping(10); // Gives about 0.02 for me - confirms that communication is working
// Now let's create a quick ring-modulator effect:
z = { AudioIn.ar([1,2]) * SinOsc.ar(MouseX.kr(100, 1000)) }.play(u)
Making standalone plugins:
You can create standalone plugins that don't need to be controlled from SuperCollider. Gerard will probably put a note here about how to do it (using a quark)...