Go to the following site:
https://gleam.io/pRBx2/free-steamkey-yargis-space-melee
Do the specific steps (requires FB and Twitter), grab the key.

http://store.steampowered.com/app/369040/
The game features trading cards.

Have fun.

Grab it at Indie Gala, in the following link:
https://www.indiegala.com/#giveaway

http://store.steampowered.com/app/345220/
Features trading cards.

Have fun!

For a limited time, Bundlestars is giving free copies of Planetary Annihilation.

All you need to do is head over the following link, https://gleam.io/7GoGe/free-steam-key-for-planetary-annihilation do the required steps (doesn't require FB or Twitter) and grab the free key.
Be warned though, due to probable high popularity of this giveaway, most likely you will face constant time-out.

http://store.steampowered.com/app/233250/
The game is a decent RTS, all in 3D, with several different planets that can stage battles, with huge armies.
It features trading cards.

Enjoy!

Go to Indie Gala site:
https://www.indiegala.com/monday#giveaway
Login with your account and claim your free game.

http://store.steampowered.com/app/257830
Features trading cards.

Have fun!

Head over to indiegala.com, scroll to the bottom half of the page and add your email on the giveaway.

http://store.steampowered.com/app/398710
Game features trading cards.

Have fun!