Popfly game engine – open source
As popfly is getting shut down August 31st, they released the game engine under the Ms-Pl. If you used the Popfly Game Downloader we released a few weeks back to grab your games, there's also instructions on how to crack them open and grab the data files. Which means you may be able to recreate the actual GUI for creating games!
Check it out over at Codeplex.
[via Ben Anderson]