2 1/2 of the makers of the L4D(2) mod Dead Before Dawn went solo and created THE CORE.
This review may contain spoilers
9/10 -
Not only does this mod have puzzles in the style of Portal 2's campaing and co-op mode, no, it also introduces us to completely new gameplay mechanics (changing gravity, a maze, etc), and all that in only 6 maps, 5 of which were excellent (I was feeling a bit "left behind" in the 6th one, it doesn't explain as much as it should do). The only thing that could be improved is that the maps' look should help the player find out the solution a bit easier/earlier (yes, you can build maps where the looks only lead to the solution). 9/10 points for a mod that really enhances portal 2's gameplay mechanics.