Simple & versatile image editor.
Cajón
Drawpile is a Free software collaborative drawing program that allows multiple users to sketch on the same canvas simultaneously.
Boonol
This application is a basic image editor, similar to Microsoft Paint, but aiming at the GNOME desktop.
In boonilo'
MyPaint is a nimble, distraction-free, and easy tool for digital painters.
Krita
Krita is a FREE and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry.