Face tracking software on mac (in build camera)

You can find a good starting point here: http://code.google.com/p/ehci/

It's a software based on OpenCV that tracks head and detect its orientation. It's opensource.

There are facetrackers implemented (and already trained with markers), for example in OpenCV. I suggest you to first start with just tracking faces. Once you have a robust facetracker, and you can generate output telling how long a face has been looking at the screen, etc.

Later you can add improvements. Once you detect a face, you can try to recognize people analizing face pixels.

Another line is to recognize parts of the face, like mouth, eyes, nose, eyebrows...

If you can track face and parts of the face, you can try to recognize facial expression patterns, like happines, sadness, etc..