I didn’t include here applications like GIMP or Krita since they are full-fledged image manipulation applications, not just simple painting programs, nor Scribus or Inkscape since they serve different purposes. This article overviews
7 8 simple drawing applications for both KDE and GNOME.
Please bare with me on the screenshots, I could never draw anything more than rectangular circles and curvy lines in my whole life.
Update: Two more applications were added to the list, Tux Paint and Pinta.