The lights are on
If achievements, trophies, and badges aren't enough for you, you'll be happy to know that Valve has introduced a new type collectible. Steam Trading Cards is now in beta, and you can start earning them in a number of Valve titles.
You can start earning them now in Counter-Strike: Global Offensive, DOTA 2, Portal 2, Half-Life 2, Team Fortress 2, and Don't Starve from Klei. Collecting a set will allow players to convert the cards into badges, which will randomly unlock an in-game emoticon, Steam profile background, a discount for a different Steam game, or 100 XP to increase your Steam Level.
Badges can be upgraded by collecting another full set of cards, earning players 100 additional XP. Levels translate into increased friend list size and additional showcase slots for favorite games, best achievements, screenshots, and more. Showcases unlock at Steam Level 10 and expand every 10 levels after that.
Half the cards will drop in-game. The other half can only be secured via trading with other members of the community. You can read the full rundown on the Steam Trading Card FAQ.