One enterprising soul has taken it upon himself to recreate the P.T. demo that made huge waves through the gaming community in the Unity engine, with an available download. As many of you will know, the demo was taken off of the Playstation Network, so if anyone wanted to play it but was unable, now is your chance.
In addition to recreating the game, a fascinating article was written about the process, and it will really make you appreciate the amount of effort that goes into these things. It is well worth a read for anyone interested in game design. Go here to read all about it, and download if you wish.
Hopefully, this will stay available, unlike when Super Mario 64's first level was recreated in Unity.