Marcos at Topsolitario offers a set of free paper models of cargo containers for your modern and futuristic miniatures games. They are quite lovely. There’s even a video showing how to build them.
The Paper Toy Construction Set is a collection of jpg images of struts and girders in a variety of sizes, shapes and angles. The idea is that you print out these pieces and then use them to build your own models — much like an erector set.
For Warhammer Ancient Battles, here is a paper model of a Roman testudo in 25mm/28mm scale.
I’m not at all sure what scale this free paper model of an elf tower is in. Its either a big 15mm or a small 25mm scale. In either case, it would be a nice addition to the table for a fantasy game.
Here’s a site with four very nice paper models of 25mm / 28mm half-timbered houses that would be appropriate for a fantasy or medieval town setting. There are “shoddy shops” a “posh place,” a plain half timbered house and a building that is more fantasy oriented. All are in pdf format.