Edit: Now expired. Thanks for the heat folks! They've replaced it with a new one, GTR Evolution - see this thread about it :)http://www.hotukdeals.com/freebies/free-gtr-evolution-steam-key-from-pc-gamer-bundle-stars-1959903
Bundlestars are continuing their free game bundle promotion - this week it's Gun Monkeys (http://store.steampowered.com/app/239450/) - it's multiplayer only, but it does have Steam Trading Cards if that's not your bag.
Requires an email, Facebook Like and allowing "Coupons" access to your FB - as soon as you get the key, remember to remove it from your FB - go to Settings, Apps, click the cross next to it.
Oh, and you actually get TWO copies of it, one for a friend :)
The site is pretty hammered right now, but they have one million to give away, so just try again in a few hours if it doesn't work right now.
Edit: Added info about the FB access.