Sketchpad: Free online drawing application for all ages. Create digital artwork to share online and export to popular image formats JPEG, PNG, SVG, and PDF.
Drawing directly in a 3D viewport makes a lot of sense. It opens unsurpassed workflow freedom for story-boarders and 2D artists. Blender has a flexible Python controlled interface.
Doodle3D Transform is a free and open-source web-app that makes designing in 3D easy and fun!