The Stanley Parable started life as a Source Mod, originally released in 2011. Since then, an expanded, standalone release has been in development for release later this month.
There is a demo, and you really want to play it.
Whilst millions of people are busily shooting each other in the face in the various incarnations of Counterstrike, Call of Duty, Battlefield and whatever else, the most interesting developments in first-person games are coming out of the indie sphere, and many don’t even involve guns.
Go play The Stanley Parable demo now, and thank me later.