Personally, I use GIMP.
Unfortunately, I'm fancy paint program illiterate so I only use it to touch up what I already draw or use it to alter photos.

gimp 4 lyf

firealpaca.

ooooh...

I myself can't stand any of the other alternatives. Firealpaca doesn't lag at all on my old desktop and has an interface that I don't find intimidating at all which is my problem with photoshop. And it is completely free and does everything I would need it to. I still favor Paint Tool Sai, but I am guessing you are looking for something even my old computer can handle that is also free. x3

I am not even sure why I hate Gimp as much as I do but I just... do. I'm dumb like that.

I don't like GIMP much, but it's more of a photo editing type program like Photoshop than SAI (and i presume Firealpaca), so I'm probably gonna keep it for photo editing. but I've never heard of Firealpaca before and I'm gonna download it because I prefer to draw/ink/color/shade/etc in SAI and it looks very similar SAI. In my opinion Photoshop (and presumably emulatory programs like GIMP) are only good for adding textures and "final touches" to artistic images, SAI is a much better overall "drawing" or painting program...