ofxFern by EPFL CVLab
ofxFern is not only useful in face recognition. It can help recognise all sorts of patterns, which inspired me of using it in my previous idea of logo recognition.
ofxMSAPhysics by Memo Atken
ofxMSAPhysics is an opensource, C++ 3D particle/constraint based physics library for openFrameworks 006+ and Cinder. Its main feature includes particles, strings, attractions and collisions. It is quite cool to make Pixar-style animation and data visualization, while the basic elements like particles and strings need to be tweaked before use.
Timeline library is used for reusing pieces from those big box applications like Aftereffects, Fireworks. It will be extremely useful when you want to add those effects in audio and video projects. What is more, data visualization will also depend on this library.