Penpot is the first open-source design tool for design and code collaboration. Designers can create stunning designs, interactive prototypes, design systems at scale, while developers enjoy ready-to-use code and make their workflow easy and fast. And all of this with no handoff drama.

Available on browser or self-hosted, Penpot works with open standards like SVG, CSS, HTML and JSON, and it’s free!

Figproxy is a tool that enables rapid prototyping of tangible user experiences allowing Figma prototypes to talk to the external world.
More specifically, it's a utility that allows bidirectional communication between Figma and physical hardware for prototyping interactions that involve screens and physical elements like motors, lights, sensors etc.
It's designed to talk to hardware prototyping platforms like Arduino.

The simplest app for Bootstrap prototyping. Create quality HTML prototypes quickly, using popular open source tools like Bootstrap, LESS, Fontawesome and more. Work visually and with sources simultaneously, with no need to export. Easily integrates into developers' toolsets (Git, text editors, etc.) Works with plain HTML and LESS files. Generate only standard Bootstrap 3.3 HTML and LESS markup.