FaceTracker is a C++ library for real-time non-rigid face tracking using OpenCV 2, and is free for non-commercial use. For commercial use, please request a quote.
FaceTracker only comes with one command-line example, but there are many other contributed wrappers maintained by their respective authors.
An addon for openFrameworks including demos of applications such as expression analysis and blink detection. openFrameworks is compatible with with Mac, Windows, Linux, Android, and iOS. An iOS example is available here.
A port of ofxFaceTracker for Cinder. Cinder is compatible with Mac, Windows, iOS, and WinRT.
An application that connects to an available webcam, tracks a face, and streams tracking information in realtime over OSC for quickly prototyping in various environments or controlling OSC-enabled applications.
An example of using FaceTracker on Android.
A Python wrapper for FaceTracker.