A while back, I created a Portal 2 custom map, Quantum Entanglement I, using a new puzzle mechanic that links two cubes together. Quite a few people in the Portal2 community ThinkingWithPortals.com played it, and it got a pretty good response.
Since Valve launched the Steam Workshop, I modified the map to run within that environment so it would be easier for more people to install and play it.
The response has been overwhelming! It was the #2 most popular map of the day yesterday! Unbelievable.
I have already started on the next installment, and I hope this will become a series of puzzles that present unique new challenges to the game.
Update: #4 Most Popular Portal2 map this week in the Steam Workshop! Over 20,000 total subscribers! Wow!