Quartz Composer is a visual dataflow environment that is similar in spirit to AVS or SCIRun or Helix for you Mac old-timers. What is especially fun about it is that "hello world" is so easy. Here is a program to feed the webcam to a preview window with a Core Image shader posterizing it:
The "clear" there is layer 1 (like a Photoshop layer) and just gives a black background. The video input is the webcam. The Core Image filter is a GLSL variant. A cool thing is that if you add arguments to the filter, input ports are automatically added.
To program that filter you hit "inspect" on the filter and you can type stuff right in there.
The code for this example is here. You can run it on a mac with quicktime player, but if you want to modify it you will need to install xcode and the quartz composer environment.