Steam Games

http://store.steampowered.com/genre/Free%20to%20Play/

https://www.indiegala.com/store
Scroll down, they're giving away a free game.

This is very useful: https://ifttt.com/recipes/164331-mail-me-free-games-with-steam-keys-posted-to-r-gamedeals

http://www.dlh.net/en/ - Create an account / login with existing account / login with Facebook, go to Profile / Keys and scroll down. You can get around 5 free Steam keys there.

https://www.indiegala.com/store - Scroll down, enter your email. You will receive Bloop. (Check back regularly on Indiegala for more free Steam keys)

http://www.lotsogg.com/ - Complete six tasks in order to receive a Steam key for Shot In The Dark.

http://www.razerzone.com/cortex-deals-nosferatu-giveaway - Free Steam key for Nosferatu: The Wrath of Malachi. Not sure of the requirements.

http://www.pcgamer.com/grab-a-free-steam-key-for-tactical-mech-game-bionic-dues/ - Free Steam key for Bionic Dues. Enter your email (May also require Facebook/Twitter).

http://www.indiekings.com/2015/10/free-steam-key-pixel-puzzles-japan.html - Pixel Puzzles Japan free Steam key. Enter your email, login with Facebook.

More detailed info and quick notifications about free games on Free Games Finders.

Thanks everyone!

Try tremor games too :-)
Just search for it

lots of free games here