Well, this is interesting. While the rest of the world was ogling over what's coming down the pipe from Google, Steam introduced collectible virtual trading cards that you can earn by playing games, and trade in for bragging rights and prizes.
Sound odd? Sure, but it's also rather neat. Steam Trading Cards are currently in beta, and themed after the few games that support them. You earn cards by playing the participating games, though you can only earn about half of a game's total card set on your own. You'll need to collect the rest of the set by bartering with friends or other Steam users.
Here's where things get interesting (and potentially valuable): complete a set of cards, and you can craft them into a game badge. These badges can be displayed on your Steam profile, but you'll also earn random goodies like backgrounds for your Steam profile and (more lucratively) coupons for DLC and discounts on Steam games.
These collectible cards can be sold on Steam's Community Market, and as of the last hour virtual game cards have been selling for about $2 - $3--which sounds ludicrous. There is of course the potential to earn a badge and get a coupon for 50% off a game or DLC, and... nope, still ludicrous. I'm sure the market prices will level off once that "shiny new feature" smell wears off, more folks get invited to the beta, and a true median price is determined by supply and demand.
The list of supported games is small: Counter Strike: Global Offensive, Dota 2, Team Fortress 2, Portal 2, Half-Life 2, and Don't Starve. Valve will be adding more games over time, but you'll need to be a member of the new Steam Trading Cards group to get in line for an invite. Good luck!