Germ’s World has a large number of 25mm modern buildings for you to download and use in your modern miniautures wargames.
Paper Models
City Walls Paper Model
Wizards of the Coast has this free 25mm – 28mm wargames model of Intact and Ruined City Walls.
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.