Martin Gumbold offers a nice paper model of a two story, European “good shed” — what I would call a warehouse, because of the loading dock. There is no apparent scale, but I downloaded it and sized it up, and think it would work very well for 20mm, and pretty well for 25mm – 28mm figures. You could use it for a variety of periods, because there are no obvious things to date it.
The Independence War Paper Models site has some very nice — and free — paper models of starships.
This free paper model of Disney’s Haunted Mansion is a tour-de-force. Even if you don’t want to download it, you need to take a look.
Germy’s World has a paper model for making your own dungeon, but what is more interesting is Victorian Cellar model that accompanies it. It’s perfect for gothic horor games as the lair of a monster, or mad scientist or cult.
Ravensblight has a free paper model of a hearse and coffins. Because you can have your printer resize these, they can be used for any number of scales and games.