https://www.indiegala.com/trades#giveaway

Not quite sure what this is, but it gives you a steam key so... here
http://www.indiekings.com/p/giveaways_6.html
entering is pretty simple, just put in your email a couple times a bam, got ya self a free steam key. It seems to be a DLC for a game that hasn't even been released on steam yet

http://www.dlh.net/en/
sign up to this site, once logged in, click Profile / Keys, scroll down to where it says "Steam games:", reveal and redeem your free steam game codes

https://gamesrepublic.com/game/strategy,anomaly-warzone-earth,123.html
Create an account then click "GET IT FOR FREE", then hover over "my profile" then click "my games" once here clcik "get it" in the bottom right corner of the game image, reveal and redeem your free game code