Join the Steam Trading Cards group on Steam to score a beta invite so you can start collecting trading cards, earning XP, and leveling up—yup, Steam just got gamified.
The games participating in the Trading Cards beta are Don't Starve, Dota 2, Counter-Strike: Global Offensive, Team Fortress 2, Portal 2, and Half-Life 2, and up to half of the new virtual trading cards for each can be earned by playing them. The other half of each set is earned "through your collecting prowess," which presumably means trading with Steam users who got different drops.
Once you collect a complete set, you'll be able to craft a game badge which will appear on your profile and unlock "marketable items like emoticons, profile backgrounds, and coupons." Badges can be leveled up by collecting the required trading cards again, and all badges—including any you already have—now give you XP which contributes to your "Steam Level." Leveling up has its own benefits, awarding you "non-tradable items like profile showcases, extra friends list slots, and more."
Now that playing games on Steam is a game, are you bothered that someone out there is already beating you? If so, the PC Gamer Steam group may be a good place to start looking for trades.