Releasing mod-tools for your game is one thing, releasing the whole engine for free so that people can work with it is another. It's a philosophy several engine-makers already follow and now it's time for Crytek to adapt it as well.
Thus CryEngine 3, the engine that powered Crysis 2, is now available as a free download. Do check the license agreement if you plan to make your own games based on it, and go nuts!