Mod DB now offers the debut release of The Stanley Parable, described as a mod for Half-Life 2, though HL2 is not actually required since it can be played using the free Source SDK (thanks BlueWolf72). Here's a trailer and here's the intentionally minimal description:

The Stanley Parable is an experimental narrative-driven first person game. It is an exploration of choice, freedom, storytelling and reality, all examined through the lens of what it means to play a video game.

You will make a choice that does not matter.

You will follow a story that has no end.

You will play a game you cannot win.

...it's actually best if you don't know anything about it before you play it :D!