Is there a popular and stable library in C# for the AdaBoost algorithm? Does such a library contain different flavors of boosting besides the classic AdaBoost (such as GentleBoost, LogitBoost, etc. )?
OpenCV supports the basic flavors of AdaBoost (Discrete, Real, Logit and Gentle). This library is in C/C++ but you can use the C# wrapper called Emgu CV although the API is more limited (mainly Train() and Predict() methods) than the C/C++ one.
You can also use Accord.Net but I haven't tried it yet.