Origami Studio : Origami is a free tool for designing modern user interfaces.
SVGOMG : SVGOMG is SVGO's Missing GUI, aiming to expose the majority, if not all the configuration options of SVGO (Nodejs-based tool for optimizing SVG vector...
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.