Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Shutter - Shutter is a feature-rich screenshot program.
Synfig Studio - Just another WordPress site
Snipping Tool - Use Snipping Tool to capture a screen shot, or snip, of any object on your screen, and then annotate, save, or share the image
Blender - Blender is the open source, cross platform suite of tools for 3D creation.