Pencil - An open-source GUI prototyping tool that's available for ALL platforms.
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.
This is a great tool! I've used it for a few years now and it does everything that I've needed. Nice find!
Yup! It's awesome. I used it years ago as well. This is an updated version, Pencil V3 that is currently being developed. You can see more details here: https://github.com/evolus/pencil/