Borderlands 2 had Steam trading card support added after the inital test batch of trading cards, but still during the beta phase, on 28 May 2013. There are 6 cards in the series, and you'll receive 3 cards at random for purchasing and playing the game.
Other entries in the series, Borderlands: The Pre-Sequel and Borderlands 3 also have trading card support.
Trading Cards
Foil Cards
Badges
Booster Pack
Emoticons
Profile Backgrounds