Valve is about to give you another reason to use its game client to play PC games, in the form of Steam Trading Cards, which are now available.
Steam Trading Cards are earned through playing games. Fulfilling certain objectives will net you a Trading Card, not unlike Achievements. These Trading Cards can, in turn, be collected and stockpiled to earn Steam rewards such as coupons, badges, profile wallpapers, and other fun swag. There’s nothing really revolutionary here, just a fun little metagame and, of course, more virtual junk to collect.
So far, about 30 titles are supported. VG 24/7 lists some of them as GRID 2, Company of Heroes 2, FTL: Faster than Light, Moncaco, Trine 2, Super Meat Boy, Sanctum 2, Magicka, Brutal Legend and Left 4 Dead 2. More will be added in time.