Gwent is an online F2P CCG based on the witcher series of books and games. It is currently in closed beta
It was a minigame in Witcher 3 and was so popular they developed it into a full blown game with proper deckbuilding, balance and mechanics.
The most obvious comparison to another game is to hearthstone. The game itself apparently borrows a lot of things from hearthstone like the rarity system and the card crafting system. I have never played hearthstone myself though so I have no idea if this is accurate or not.
The game itself is really interesting. Gwent is mostly about bluffing your opponent and managing your resources to take a best of three match. Instead of resetting both players decks in each round and starting from scratch , like for example in Magic, the cards you draw from your deck at the start of the match are the ones you keep until the best of three is over. You get to draw two cards at the start of the second round and one card at the final round if it goes there. To win a round you need to get a higher total than your opponent through the use of unit cards. Each unit card has a corresponding strength attached to it and these get totaled once both players have decided to pass the round and stop playing cards. Its very important to note you HAVE to play a card in Gwent and will be forced to pass otherwise. Cards left on the board are destroyed after a round, so you need to be careful you do not overcommit too many resources to win. Of course, the game is much more complicated and some cards can do some crazy shit to affect the board, but that's the basics.
I have been playing this for something like 3/4 hours a day for the past couple of weeks and honestly loving it. Even know it is F2P I find the reward system to be quite reasonable and you are able to be competitive fairly quickly. You get cards from "kegs" and you can easily farm 3-4 a day with a couple of hours of gameplay thanks to the daily missions. You can also "mill" unwanted cards and craft new ones.
You can sign up for the closed beta here: https://www.playgwent.com/en/faq
Some gameplay / tutorial footage: