So, https://duelyst.com/
It looks a little goofy in it's promo shots, it's name is a little cringy, but man oh man the gameplay and strategy is strong.
In comparison to Hearthstone, you have your character that represents you on a grid board, a hand of cards, and creatures with a taunt skill. Thats about where the similarities end. Since everything is on a grid, you can body block units with some of your own, some units fire at range and should be abused as such, units with flight can move anywhere they want on the board which can set you up for more moves. It's really good. And the pixel graphics are really neat too...
As for the free to play aspect, it's really generous with 'gold', there is an arena just like Hearthstone, and you can get into it with paying nothing. A lot of tutorial and single player challenge 'puzzles' give gold to help you get started with a starting wallet of gold that you can buy packs or play the arena with. I always do the arena cause you get a pack if you win or lose, plus more.