Robot Battle - An evolving programming game that allows players to design and build intelligent robots using a simple scripting language. - http://www.robotbattle.com/
Ladder - A classic ASCII based arcade game similar to Donkey Kong. (Requires the Java runtime environment.) - http://ostermiller.org/ladder/
Adventures of Johnny Jetpack - Guide Johnny through his 3D world of platforms and lava to collect all the coins. (Open Source) - http://www.johnnyjetpack.net/
The Adventures of Jason - Java platformer in which one or more players jump around in a rectangular world, trying to catch stars and shoot each other. [Open Source, GPL] - http://javagame.sourceforge.net/