I’ve only briefly dabbled in Portal 2 Mapping, but in that time I created a map with a unique gameplay mechanic: Portal Buttons, a system where players can activate items by placing the game’s “Portals” on specific marked surfaces. These can be activated from far away easily, but then restrict the player’s movement while the portal is activated.
This adds a whole new puzzle element to the Portal games, which can be combined with others into deeper, more layered puzzles. In particular the buttons restrict the player’s ability to use their portalgun, and gives the players a mechanic they can interact with in unusual ways (placing a portal so as to replace one on a button and deactivate the button, for example, which can be done even from a different room).
Portal Buttons was an entry to the ThinkingWithPortals.com Portal 2 contest: it placed highly and was praised for its innovative design and lavish detailing. It can be downloaded here. I also made two other maps for Portal 2 using the Perpetual Testing Initiative tool – they can be found here and here.