Epic made an Unreal Development Kit. And it's absolutely free. This means, you have the whole U3 Engine in your fingers. It's especially interesting for indiedeveloper and modders, since they now can make a stand-alone version of their game and even sell it! With a license for 99$ you can sell your game until you reached the 5000$ mark. Then you have to pay 25% to Epic or buy a better license

I think this is fantastic
Any chance of a Star Wars mod in the U3 engine? Of course, you can't sell it, but still

http://www.udk.com/ for more infos