I'm wondering what software most people on this board to create their 3D "worlds". I've looked at FreeWorld3D 2.0 and 3D World Studio; both look nice, but I'm curious if most people just roll their own, or use some other software I haven't seen. I'd like to create a small world that I can use to learn 3D engine programming a little bit better. Also, do most people here use .X files or another format?

In a commercial setting most people roll their own unless they are working with some big middleware product (i.e. Unreal or Renderware)

There has been some efforts to have people running Maya, Max, XSI, etc as their world editors, and currently most of those have some kind of support for embedding a game viewer, but I have not seen it used much in practice due to the licensing restrictions (Requiring a license for each designer)