OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Synfig Studio - Just another WordPress site
Mediamodifier - Create beautiful designs and product mockups in seconds.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
mockDrop - Is a webapp for creating free device mockups including your screenshots.