For a long time I was a big fan of papercrafts. I've still got several bins full of little paper minis and terrain stashed away. I've since moved to using Maptools as a digital tabletop. However, papercraft is still a lot of fun and can be a great tool for people who like physical minis. I've been browsing papercraft collections and I figured I'd provide a set of links to a various great RPG-able (and free) papercrafts out there:
http://minipapermodels.blogspot.com/ This is a papercraft blog dedicated to RPG minis and it has some really great stuff, the latest being a set of elementals (I especially like the air elemental).
http://www.onemonk.com/downloads One Monk has some really great miniatures and a terrific gallery of free papercraft models. The Cthulhu model (pictured) is one of my favorites but they also have a great dwarven steam tank and a purple worm that are equally impressive.
http://www.combatstorm.com/download.html Some modern buildings that work great for military games, superheroes or urban fantasy.
http://www.germy.co.uk/fprpg.htm Some simple, easy-to-make and quick dungeon and structure models.
http://www.wargamevault.com/index.php?filters=0_40209_0&free=1 a good selection of random, free terrain/buildings
http://www.buildmodelcastles.com/html/castles.html castles built from toothpicks, wood and paper
http://www.wizards.com/default.asp?x=dnd/fpm/archive some really high quality buildings from WoTC
http://papermau.blogspot.com/2011/06/deskto... a huge and very impressive castle with plenty of other material throughout the site
One of the best general resources for papercraft on the net is http://papercraftparadise.blogspot.com/. It's not RPG specific but it's a great source of models and is updated with new material constantly.
Of course if you're looking for really high quality papercraft models and are willing to pay a bit then http://www.worldworksgames.com/store/ is probably the biggest and best papercraft store I've run into.