The Geomajas API project contains some annotations that can be used to indicate which parts (which classes and which (public) methods or fields) are considered as the API. This is particularly useful for projects where you want to mark the API without the need to refactor everything to interfaces and factories.
Funcito is a Java library which simplifies access to your favorite functional programming APIs by wrapping Java methods as the function-type objects defined in those libraries. Rather than creating another functional programming API, Funcito makes it easy to define function-type objects which wrap existing methods. As a result, your code has less noise and clutter of anonymous inner classes, annotations, etc. Your code remains safe for automated refactoring, since there are no String-based reflection or forced naming conventions. The supported frameworks are Google Guava, Jedi, Functional Java, Play! Framework 2, and collections-generic.
Protégé provides a suite of tools to construct domain models and knowledge-based applications with ontologies. At its core, Protégé implements a rich set of knowledge-modeling structures and actions which support the creation, visualization, and manipulation of ontologies in various representation formats. It can be customized to provide domain-friendly support for creating knowledge models and entering data, and can be extended by way of a plugin architecture and a Java-based API for building knowledge-based tools and applications.