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.
Phillip Fickling has a paper model of a submarine that may be of use for victorian science fiction fans. Click on the free model link.
This three story european paper building model looks like it would work pretty good for 15mm miniature wargames. Players of Flames of War should take a look and see if it fits in with their tabletop layouts.
The Paper Model directory has a collection of links to dozens of paper car models, from DeLoreans to Hondas. These are just what you need to populate the streets of your modern era miniatures wargames. As is usual with these, you can use your printer settings, graphics program (that’s what I use) or a copy machine to adjust these to whateve scale you need.
If you’ve wanted to try out some of the outstanding western games that are in print these days, but don’t hvae the figures, you could download and print these sheets of 19th century western gunfighters, animals and other assorted things.