The TopSolitario website has a paper model of a science fiction Mobile Army Surgical Hospital (MASH) container unit. Its designed for Infinity, but would be useful for any science fiction game.
Ravensblight Manor is a free paper model of a haunted house. I took a look at it, and think that it would be just about the right size for 25mm – 28mm figures.
Here’s a nice set of paper model walls, columns, corners and stairs for constructing your own Mines of Moria.
Genet Models offers two free paper models for starfighters and a model of an orbital missile gunboat.
For your jungle games, here’s a paper hut model.