For fans of the wildly imaginative Miyazaki animated films, here’s a paper model of Howl’s Moving Castle. It might have some use in a fantasy or steampunk game; otherwise, it just looks like it would be fun to build.
Haunted Dimensions has a free paper model of the spooky entrance gate outside Disney’s Haunted Mansion.
The Haunted Dimensions site has paper models of three small crypts. The only downside of these is that the back of the crypts are defaced with site advertising. Still, there’s also a brick wall there that you can paste the crypts up against.
In N scale here are some free paper models of the tracks of Chicago’s famous “L”.
For Warhammer Ancient Battles, here is a paper model of a Roman testudo in 25mm/28mm scale.