The dungeon crawling role playing game, Legend of Grimrock, has just added Steam Workshop integration to their game along with a new dungeon editor. Owners of the game can now browse the Steam Workshop and easily find custom dungeons to add to their game. Testers in a Public Beta have already created over 150 mods that can be downloaded via the Grimrock Workshop.
For users that would like to try their hand at creating their own dungeons, the developers have put together a wiki here with tutorials and resources.
Please see Legend of Grimrock on Steam to buy the game and for more information.