A user named Steve_Big_Guns has is creating an FPS called Massive Epic Raging Destruction using the creation tools in LittleBigPlanet 2. Seriously, the ingenuity of the game community never ceases to amaze.

As you can see in the video below, the game seems to be running fairly well, if a little stiffly. Still, considering this was made with off the shelf creation tools in a commercially released game, it's really an impressive feat.