The available tools for game development are better than ever and widely available. The problem is that users expect AAA title quality, and that's a big job.

If you want to develop inside an existing environment, there are the virtual worlds - Minecraft, SineSpace, High Fidelity, Sansar, and Second Life. Even 3DXchat, which is supposed to be porno but has enough building tools that people are using it for other things. SineSpace is very developer friendly right now. Few users, but nice tools.