Storyboarder - Storyboarder makes it easy to visualize a story as fast you can draw stick figures.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Storyboard Quick - Professional storyboards made in minutes: No drawing necessary!
Sketch - Professional digital design for Mac.
Boords - Making storyboards can be fiddly.
Affinity Designer - Professional creative software, exclusively for Mac.