Spring Framework * - a full-blown enterprise capable Java framework, based on patterns like dependency injection and providing an advanced bean container and utilities.
Apache Struts * - the Struts framework encourages application architectures based on the Model-View-Controller (MVC) design paradigm, and is targeted at Servlet and JSP based applications.
Poolman object pool * - provides a means of pooling and caching Java objects, as well as extensions for caching SQL queries and results across multiple databases.
JavaWorld "Build your own object pool" * - article about increasing the speed of applications while reducing memory requirements.
JUnit * - a regression testing framework written by Erich Gamma and Kent Beck.