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 10 simple drawing applications for both KDE and GNOME. The command to install each of them in Ubuntu is displayed below the screenshot of the program.
Please bare with me on the screenshots, I could never draw anything more than rectangular circles and curvy lines.
Update 1: Three more applications were added to the list, Tux Paint, Pinta and GrafX2.