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.
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.
Delta 7 Studios has a collection of free paper models of the Seven Wonders of the Ancient World: the Great Pyramid of Giza, Hanging Gardens of Babylon, Temple of Artemis, Statue of Zeus at Olympia, Mausoleum of Maussollos, Colossus of Rhodes and the Lighthouse of Alexandria.
Dungeoneering dot Net has a nice selection of printable terrain tiles ostensibly for use with the Dungeons and Dragons miniatures game, but which would also work with any number of dungeon crawl games — of which there are many free ones available on this site.