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.
IRC-API is a stateful, asynchronous IRC API written in Java. It offers an (a)synchronous programming model to its user, and has the ability to save the IRC server state per IRC connection. It supports SSL. The API has been tested on various IRC networks, including EFnet, DALnet, and Undernet. It offers a useful, detailed interface to the Server it connects to.
OpenStack4j is an OpenStack client which allows provisioning and control of an OpenStack system. This includes support for Identity, Compute, Image, Block Storage, and Network using Java. The library supports the following OpenStack services: Identity (Keystone), Compute (Nova), Network (Neutron), Image (Glance), and Block Storage (Cinder).