https://www.facebook.com/MetroVideoGame/
If you like this page and click on the Free Download section you will get a free Metro 2033 key for use with Steam. It's pretty awesome if you didn't already get the game itself or the THQ Humble Bundle.
Alas, I've had this game in my Steam library for a looooong time.
Couldn't even take advantage of the bundle...already had all the games I wanted in it.