FireAlpaca!!! It is for free and is available for both MAC and Windows. And it is really good (mostly since it is a free software). It is more for art, so it has that brush stabilizer thing (I love that), and the paint actually blend, I think that's really nice =33 (but the text tool is horrible)
I use it for sketches and paint bucket, it might sound "Really, paint bucket? That's stupid.", but they have THE BEST paint bucket o-ó No kidding, it fills so perfectly (with 1 in the expand box, but way better than Clip Studio and Photoshop for that purpose)
There is also Krita, for free. I didn't use this one really much, since FireAlpaca and Clip Studio Paint are working just fine for me. But it is worth trying.
Clip Studio Paint, it is not a free software, but on sale it is almost a free software (I paid 15USD on the Pro version), and an AWESOME one. No kidding, their brushes/pen/pencil are amazing, the ones that feels most real to me, and they have this perspective rules that are heaven, seriously, Manga Studio / Clip Studio Paint is the most beautiful thing for perpective drawing. They have a trial version, TRY IT! o-ó (fangirl)
My inker always told me to use Manga Studio, and I was like "but I don't wanna learn... And I hate cracking things", but then I tried Clip Studio Paint and omg she was right, she was so freaking right, I heart Clip Stduio.