Gwent is an online free to play card battle game based on The Witcher[Wiedźmin] universe. It was first introduced as a mini game in The Witcher 3: Wild Hunt[Wiedźmin III: Dziki Gon]. Players must win 2 out of 3 rounds to win the match.
Depending on current patch this can be anything from a 6/10 to a 9/10, but in general not being throttled by a mana system allows for incredible freedom and some truly zany play styles that can find moderate success even at pro rank. It's still the best f2p card game on the market I'm aware of.