there are pretty fully-powered editors that come shipped with Unreal Tournament 2004 and Oblivion 4. These are some of the same tools that were used in creating each of those games. For the price of the game, you get access to the creation tools, as well as the mod community for any assistance that you might need in getting acquainted with the two editors.
And beyond that, there are a number of free, open-source engines out there with documentation and communities of their own. Search around for some game engines, you're bound to come across something