The Stanley Parable, Half-Life 2 Mod that was just Greenlit by the Steam Community, is the game you should be downloading right now (if you haven't already).
Their description of the game:
What is The Stanley Parable: HD Remix?
It is the remake/sequel/expansion to The Stanley Parable, a Source mod released in 2011. The game is an experiment in video game storytelling, or the point of it might be that thereís actually no story at all, or maybe the point is that thereís no game. Itís about choice and freedom, possibly, but actually it might be about something else. You will make a decision that means nothing, you will walk a path that goes nowhere, and if you REEEEEEEEALLY look hard enough, none of it will make sense.
If this article from ArsTechinca doesn't convince you to give it a try...
...then their sardonic video response to a fan's email should do the trick.
Check out their Steam Greenlight Page for more goodies:
(the screenshot gallery is especially amazing.)
Or just go download it right now (it's free!): http://www.moddb.com/mods/the-stanley-parable