Not sure if this counts as "Terrain" for the purpose of this discussion, but I love these things:
Hirst Arts Castle Molds
They're rubber molds that you can use to cast gaming structures using cheap plaster of paris or slightly more expensive hydrocal (Trust me, you'll want to go with Hydrocal for reusable terrain- look for it at art stores). I make specific structures for specific games, and 1"x1"x4" or 1"x1"x6" blocks to rough out interior spaces quickly. I've found I can rough out a dungeon room much faster using blocks than I can using markers, and you get less marker on your hands that way.
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