Vote in the Golden Joystick Awards, and you can get a Steam code for BioShock Infinite for $1USD/£1/€1. The process is a little long and convoluted, potentially involving 2 new accounts, linking your Steam account, waiting on e-mails... Here's the full promotion terms from GMG. I've got a few extra tips below.
https://goldenjoysticks.greenmangaming.com/free-games-terms
One thing to note before beginning is that GreenManGaming's system is dopey, and will require you to have the same e-mail address across different services (GamesRadar, PayPal).
1. Register/login at GamesRadar. If you can, and you already have a GreenManGaming account, use the same e-mail address. Otherwise, you'll have to change things on the GMG end later.
http://www.gamesradar.com/
2. Vote for various categories in the Golden Joystick Awards. You don't have to vote in every category; there are navigation arrows that let you skip. There's a "Best Gaming Moment" category (#11 of 23), which supposedly contains some pretty major spoilers. I skipped past it, so I couldn't even tell you which games are involved.
http://www.gamesradar.com/goldenjoystickawards/vote/
3. Wait for an e-mail from GamesRadar, confirming you've voted. I got mine within a few minutes. It should direct you to...
4. https://goldenjoysticks.greenmangaming.com/
Enter your info on the GreenManGaming side. If you don't have a GMG account, use your GamesRadar e-mail address.
If you already have a GMG account, login on a separate page, check the account settings and make sure your e-mail address is the same as you used on the GamesRadar side. Then enter your account info on the "Claim Your Game" page, including user name & password. (It makes it seem as though you're creating a new account, but existing account info will work.)
5. Wait for another e-mail from GMG, asking you to verify your e-mail address.
6. You may have to sign in to Playfire (using GMG credentials) to link your Steam account to a Playfire account. I had previously done this, so I don't know if another e-mail will instruct you to.
7. Wait for another e-mail from GMG, containing a voucher code. This will allow you to buy BioShock Infinite from GMG for $1. If paying with PayPal, once again, your GMG e-mail and PayPal e-mail will need to match.
8. Redeem your BioShock Infinite code on Steam.
9. If you like, play BioShock Infinite for the first time by October 16th to receive $1 back in Playfire Rewards--redeemable on a future GMG purchase.
https://www.playfire.com/a/rewards