It’s called Krita. Maybe you’ve heard of it, but in case you haven’t, seriously, check it out! It’s really nice!
The best way I can describe this program is that it’s a combination of Easy Paint Tool SAI and Photoshop. And the best part is the program is completely free to download from their website. I think what I like most about it is that not only does it have a Stablizer like SAI does, but it has practically all the tools I ever use from Photoshop. Not saying it’s going to replace SAI or PS for me (at least not yet; I’m still learning my way around Krita lol), but it’s probably the only thing I’ve ever found that is coming close to doing it.
IT DOES WORK ON MAC IM USING IT RIGHT NOW AND ITS ACTUALLY PRETTY NEAT I DONT USUALLY ADD ONTO POSTS LIKE THIS BUT I FELT THIS WAS IMPORTANT BC 999999% OF EVERYTHING FUCKS UP ON A MAC BUT THIS WORKS AND ITS FREE AND ITS SUPER NEAAAAATOOOO
.. as u can see im not even an artist. but. yknow. A+