My Java util library. It contains many variations of Enumerations, Iterators, Collections, Sets and Maps (e.g. Map2D, Map3D, InvertibleMap, SetMap). It also contains very nice Model-View-Adapter framework. Necessarily see the AnotherClass interface. It will blow your mind.
Furthermore it contains interfaces like: Hierarchical, Factory, Converter, Overridable, Persistable, Bindable, Sequential and many other.