Valve recently announced the Steam Trading Card Game which uses a combination of time played and your skills in those games to give you collectible cards. When you have all cards in a set you can trade them in for special rewards, each set gives you a badge and a tradable item like backdrops or coupons for in-game purchases. You can trade your cards with your friends to get a complete set and each set can be completed more than once for even better rewards.
Cards can only be achieved for games you have paid for (or someone else gifted to you), meaning in free-to-play games you need to make an in-game purchase, at the moment the minimum purchase to get cards from that game is $9 USD. On the other hand, for each $9 you spend in those games, you will get one card.
This is still in beta, so not all games are eligable to drop cards, there is an official list of the current 42 games participating here.
You can see what cards you currently have on your steam profile, click on Badges and there should be a new section there for trading cards.
For more information, check the Steam Trading Card Game page.
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