https://www.humblebundle.com/store
You can get a copy of Metro 2033 for Steam for free for the next like 21 hours or so.
Did I mention it's free?
Edit: Middle of the page, big announcement, hard to miss.
Put e-mail, get Steam code (You'll have to access steam through your browser and give permissions to HumbleBundle). You then go to your library to instal the game.
Edit 2: An IRL friend said he had to imput his mail twice before he got the code (He got it a few seconds after he put his email for the second time).
Edit 3: Using GMail seems like the way to go (It's a Google con