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.
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.