macos
Pencil: An open-source GUI prototyping tool that's available for ALL platforms. (Free software)
Pencil is built for the purpose of providing a free and open-source GUI prototyping tool that people can easily install and use to create mockups in popular desktop platforms. Features * App mockup * Built-in shape collections * Diagram Drawing * Export to several format * OpenClipart.org support * Inter-page linking * Templates Stencils * Android * iOS