Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Storyboard That - Storyboard That is the world's best online storyboard creator.
draw.io - Online diagramming application
Storyboarder - Storyboarder makes it easy to visualize a story as fast you can draw stick figures.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Boords - Making storyboards can be fiddly.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.