Inkscape is a free and open-source vector art editor that uses Scalable Vector Graphics (SVG), allowing your artwork to be scaled to any size without losing quality.
GIMP is a free and open-source raster graphics editor used for a variety of graphics editing tasks such as photo retouching, image composition, image authoring, and even drawing and painting.
MyPaint is a free and open source digital painting program developed by Martin Renold that includes an unlimited canvas, a custom procedural brush engine, and a diverse set of color wheels.
Blender is an all-in-one free and open-source 3D modeling program that allows you to model, sculpt, rig, animate, run physics simulations, render, composite, and even draw and paint all in one application.