AzPainter

2025 Edit: Coming back years later to mention that this is now available as an Appimage on Linux. There is no need for a complicated install process anymore; just download the Appimage, make it executable as a program (right-click - Properties, in your File Manager), and double-click on the Appimage to run it! Should work great on pretty much any distro!

Old Comment: Installing this requires that you run a few commands in Terminal, but thankfully the readme file does show a list of required packages that you should be able to install right from Terminal if you're running a distro with the right repositories. I was able to install this in Kubuntu 15.10 by grabbing those packages I was missing via Terminal, and following the installation instructions.

Once you get AzPainter installed, it's very easy to get used to and quite user-friendly. It has a set of tools and a GUI that remind me of Paint Tool Sai and Fire Alpaca on Windows. Overall, a great drawing and painting program with pressure-sensitive tablet support (Wacom, in my case). It's lighter on resources and more straightforward than GIMP, which makes it easy to just jump right into and start drawing. It can output files in PSD format, making it easy to move over to GIMP if necessary. Hopefully this will show up in the various Software Centers eventually because it's really something that all artists on Linux should try out.