http://code.google.com/p/script-blocks/
http://www.youtube.com/v/gb7hImbSOa0
http://www.youtube.com/watch?v=V7VazBaf-zI
This is done by a friend of mine, it's sort of a minecraft/boomblox cross sandbox game. Right now you can just build cool structures, but he's working on adding challenges/puzzles.
Right now the game is entirely open source along with the art and it's easy to mod in new blocks (they're all scripted in javascript, hence scriptblocks).
It runs in Java so it's cross-platform and it uses the jMonkey engine for rendering. Also it's be developed in Ubuntu (I had a little help on it early on).
His blog along with more videos:
http://scriptblocks.blogspot.com/
Anyway, check it out and feel free to post it on the blog. He's getting a little discouraged since there seems to be no feedback.