Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Botsociety - Botsociety is a tool to design, preview and export your chatbot.
Sketch - Professional digital design for Mac.
Balsamiq Mockups 3 - New public beta for the popular prototyping tool
Affinity Designer - Professional creative software, exclusively for Mac.
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform