There is a lot of talk about 3D Model design applications. How about a level editor? Something appropriate for interior/exterior architectual design, of the likes seen in games such as Quake or Unreal. I've got my hands on a program called GTKRadiant (free) that does this, but I can't for the life of me figure out how to export to a Unity friendly format. Did anyone successfuly use GTKRadiant with Unity? There is also SketchUp, but the Pro/Commercial license is out of my range right now. Can anyone recommend any other programs?