I like Krita, it has SOOOOO many awesome premade brushes in it and its a lot like Photoshop except free.
https://krita.org/

There is also Fire Alpaca and its similar-ish to Sai but a pretty simple program.
http://firealpaca.com/en

I...I use Sai. xD

Krita! It's awesome, WAYYY better than SAI (Even though I LOVE SAI), but!!! One feature request needs to be boosted! https://bugs.kde.org/show_bug.cgi?id=342435 Cause no natural material is the same shade throughout itself, there's always a bit of hue, saturation or value variance at random.

I've been hearing more and more about Kritta lately, I gotta check it out O:

Fair warning, it's got a very steep learning curve. It's unlike Gimp, SAI or Photoshop. The biggest annoyance? The tweaks to each brush are NOT saved if you switch tools. For example let's say you spent five minutes adjusting the pressure curves and effects to a brush, and then click on a different brush...you just lost all those changes. It's annoying, so save your brushes after tweaking!

If one wants to compare it to anything out there, the closest workflow-wise is probably Corel Painter... But yeah, it's very much its own beast :) As for annoyances - report all the things! :D If it can be fit into the workflow in a sensible way, it will very likely be added (because the devs are just cool like that ;) )

I'm a Linux user, so I use GIMP.

Krita is for linux, too :D Using it right now using archlinux and KDE.

Well, that works out too.

Just had trubles with it not fitting to my screen.

If you mean vertically not fitting, there is (or was, honestly not sure if it's still there) a bug which caused the toolbar which is default docked on the left side to end up one column wide, which makes the window rather tall... Other than that it ought to fit fine on pretty much any screen (seen people use it on netbooks before ;) )

That was exactly my problem. I searched everywhere for a way to fix it, but I guess I was just too stupid to change just that. :P
If that fixes my problem, then I'll make it my primary drawing program. It had really good brushes, that I loved. :P

I used to work with Photoshop AND Sai but since I have lost my hacked PS' Exe file. . I been leanning more on SAI.

Hey, at least SAI 2.O will be out soon. _{I hope it's fucking good.}