Germ’s World has a large number of 25mm modern buildings for you to download and use in your modern miniautures wargames.
It’s the strangest thing: 25mm – 28mm scale paper models of …. books. No. Really.
Stoelzel’s Models offers a free paper model of a derelict commercial building.
The Illinois Historic Preservation Agency has a great collection of paper models of historic Illinois buildings. You can put together a very nice early 20th century mid-western town from these models.
Genet Models offers some free paper models for Dropships, Shuttlecraft, a Science Fiction Tank that looks like it is from the movie Aliens, and some science fiction style warehouses.