Scikit Learn comes to mind - not just because I can dig into the source code and immediately know what's happening, but also for the stellar documentation that

Scikit Learn comes to mind - not just because I can dig into the source code and immediately know what's happening, but also for the stellar documentation that goes above and beyond telling your what the functions do. For example their Cross Validation documentation is amazing: http://scikit-learn.org/stable/modules/cross_validation.html#cross-validation-iterators http://scikit-learn.org/stable/modules/cross_validation.html...