Some of my (very rough) High Frontier models are shown on this page;
http://www.orionsarm.com/civ/Habitats.html
The other, non-Celestia images are by NASA and Anders Sandberg. Of course a lot of people use the NASA images because they are public domain.
I haven't got the time at the moment to make new and better ones, so I am looking forward to Chuft-Captain's models.
My models are really only good for long distance shots; they doon't rotate, and have no detailing. But they are included in the OA complete add-on anyway, to give an idea of what sort of structures would be present in a mature interstellar society.