Gimp
Free & open source image editor similar to Photoshop
Best
Gravit Designer
Free full-featured vector graphic design app that works on ALL platforms
Blender
Open source, free animation, 3D modeling, etc.
Raw Therapee
Cross-platform raw image processing program
Krita
Sketching and painting program designed for digital artists
Adobe XD
Free design tool from Adobe
Best
Lunacy
Sketch for Windows
Responsively
A tool for designers and frontend developers to design and debug their in all platforms with ease
Darktable
Free & Open source photography workflow application and raw developer
Inpixio photo Editor
Free Photo Editor: For windows only
Eagle
Organize all your reference images in one place
Google Web Designer
Create engaging, interactive HTML5-based designs and motion graphics that can run on any device.
Origami Studio
Interactive interface design tool created by Facebook: For mac only
