Wizards of the Coast has this free 25mm – 28mm wargames model of Intact and Ruined City Walls.
paper model
Paper Medieval Cottages
Wizards of the Coast has this 25mm – 28mm papermodel of a medieval cottage. You could print out a whole bunch of these and have a whole village for your miniature wargames skirmishes.
Paper Models Of Science Fiction MASH Container Units
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
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.
Mines of Moria Paper Model
Here’s a nice set of paper model walls, columns, corners and stairs for constructing your own Mines of Moria.