TerrainCrate by Mantic Games is in the last few hours of it's campaign. As you can see, there are offering a metric ton of terrain options, from Dungeon and Battlefield accessories, to add-ons from their popular Dungeon Saga product and a Dark Wizard's Tower pack suitable for any Dungeon or Castle sets you may already have.
They've been blowing through stretch goals the whole campain, and look to be on target to raise about half a million.
Regardless of what you might be looking for, I'm sure you can find something usable in the many different offering in this Kickstarter. Check them out today!