my google searching effort for cool Java open source game stuff has paid off. Very cool enough that I'd make a posting for it here.
First off Pulpcore http://www.interactivepulp.com/pulpcore/
This demo says(IMO), "Java Applets can crush Flash games now". Really cool demo, looks like Doom for kids!
Next off is, Robocode, http://robocode.sourceforge.net/
A game for programmers where you create a Java code for a robot to battle! nice! Every thing is up and running without any pain in setting up, they got good docs too. Here's a screenshot.