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.
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.
Add a creepy cemetary to your tabletop with this paper model from Ravensblight. The file is a pdf, so you cna resize it using your printer's control panel to make it fit whatever scale you need.
Here's an interesting concept: Truetype and Type1 Fonts that are sets of wargaming and roleplaying figures instead of letters. Because they are, like all fonts, scalable, you can print them at whatever size you prefer.
Among the font sets are superheroes, pulp adventures, space opera, Ironclaw roleplaying figures, treasure, scenery and starships.
Here's a paper model of a medieval watchtower in 1:140 scale. The site is in German and the babelfish that I used wasn't very good in translating it, so I can't tell you much more. 1:140 scale is probably not good for most miniature wargames, but you could easily scale them up when you print.