Not sure if this was allowed as there is a Winter Steam Sale post which is great, however I saw nothing getting highlighted about FREE TRADING CARDS probably worth a lot more at the beginning to sell on the market than later on. Simply discover games through the queue and get three free cards everyday. Basically just keep clicking next and keep getting a new queue for the three cards. This will be for the entirety of the Winter Steam Sale from what I can tell.
Hopefully we can make a quick $1 or $2 from selling those cards ASAP! Or keep em if that's your thing.
There are a bunch of cards that haven't been even listed on market yet, so foils and other "Mysterious Cards" are available to be redeemed and PUT YOUR OWN PRICE ON IT! Just try and get them sold as fast as possible though, since these things usually degrade so fast on the Steam Market.
Please take this down if it's listed as a duplicate, not 100% sure.
NOTE: YOU ARE REQUIRED TO BE LEVEL 5+ ON STEAM