This has been free before but before FB was required whereas now it isn't. The game has trading cards. The usual requirements for Indiegala giveaways apply:
http://store.steampowered.com/app/284930/
How to get the key:
IG has now got some requirements to be fulfilled before you can get the free key. Those are:
Steam account linked to your Indiegala account
Steam account is a member of the IG Steam group
Steam account must be set to public
The procedure to get the key itself has remained the same:
click on Claim Giveaway button
log in to your IG account
do the captcha
click on the "Claim Free Copy" button
do captcha again (if necessary)
claim your key in your IG account (under IG Giveaways).
redeem key in Steam client
Enjoy!
EDIT: After you got your key you can get another free Steam key for Blockstorm if you FB as a FB login is required (thanks to dfaktz for pointing this out - did notice that but due to FB did not want to bother but since it is has been mentioned now I better include it. ;-))
Once you are logged in and you click the redeem button, don't click through too fast, you may miss this extra free key for 'BlockStorm' (It has trading cards and a Mostly Positive Steam rating) http://i.imgur.com/g28U1Ev.png
(Note: Would seem that it requires a simple FB login)