Worthy of further study:
Webwork is an MVC framework along the lines of struts, but smaller and cleaner, with less servlet-specificness.
Xindice is a native XML database that allows the use of XPath for queries.
Kodo is a JDO implementation that makes persistence look really easy.
Sitemesh is a dynamic decoration framework for HTML pages.
Lucene is a java based document indexing system and full text search engine.