Hewlett-Packard's Semantic Web team has been quietly working on Jena?a full-featured Java API for RDF?about as long as work has been progressing on RDF itself. In fact, the cochair of the RDF Working Group is Brian McBride, one of the creators of Jena.
Jena is an open source API and toolkit, accessible at Source Forge (http://sourceforge.net/projects/jena) or at http://www.hpl.hp.com/semweb/jena.htm. In addition, there's a Jena developers' discussion forum at http://groups.yahoo.com/group/jena-dev/.