FreeGameDev

Pretty awesome project as a spin-off from the unofficial Blender Armory Game Engine:
http://armorpaint.org/

Here is quick start video:
https://youtu.be/5YIvj3yIP00

Source code here: https://github.com/armory3d/armorpaint

But please support the development by buying it on itch.io or supporting their patreon.