Mitch’s Wargaming and Model Making Blog has a short illustrated article on making an English Civil War style house. I’ve used similar techniques to build a town of houses for my Gothic Horror games.
Osprey is supporting their popular Frostgrave rules with some paper model ruins for the game. They could of course also be used for a number of other medieval and fantasy games.
Here is a photo tutorial on making wargames buildings from foam blocks and balsa wood.
Mercerius Atticus has created some very impressive flat buildings. This isn’t a how-to, per se, but the idea is worth passing on. These would be especially useful on the edges of a game.