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.