The reference implementation of the JavaMail API.
Writing WebDAV enabled services is simple using "WebDAV Support for JAX-RS" in conjunction with "WebDAV Interoperability Filter". Check out the example project "WebDAV Address Book" to learn how this is done in an 100% vendor-independent way!
Tool for synchronization of wiki content with another wiki.
API and UI to develop wizards in Swing easily
Simple but correct tree-based XML API
A Library of Java code, which I use in several projects.