As of today, the Steam Summer Sale has kicked off, and in addition to the usual fare of a lot of games for not a lot of money, they've introduced a little meta-game to the service in the form of trading cards (http://steamcommunity.com/tradingcards/faq/).
Basically, if you are playing a game through Steam you have a chance of receiving a trading card. You can sell these for steam wallet credit through the market, essentially turning your game time into store credit. You don't even need to play it, leaving it idling in the background is enough. That's where the story ends for those of us who are only interested in free (pretend) money.
If you complete a set of trading cards, you can craft them into a badge, giving you emoticons, profile backgrounds, or coupons, as well as experience points. These XP go into your Steam profile level. You can craft badges up to five times using the same set of cards for more rewards. Aside from crafting badges, you can increase your level by participating in events and enlarging your games library.
There are also booster packs. These contain 3 cards, and they are given away whenever someone crafts a badge. Your Steam level determines your chance of getting a booster pack. Every ten levels, your odds are increased by 20%.
Booster packs, as well as regular card drops, have a chance of yielding 'foil' cards, which are rarer (and thus more valuable) than normal cards. A set of these crafts you a foil badge, which gives you the regular rewards, and bragging rights.
You can tell Valve's resident economist is having a field day with this.
So far I've gained about €10 in Steam bucks that I can put towards new games, and exhausting all of my games for card drops took about three days, during which I had them running in the background. Not too shabby, if you ask me.
So, anything you'd like to get from this sale? Remember that if there's something you're planning on buying, don't get it until it shows up in a daily deal or a flash sale, and get it on the last day otherwise.