Turn your Google Sheet into an API - Prototypes from sheets.

Thousands of people are using Sheety to turn their spreadsheets into powerful APIs to rapidly develop prototypes, websites, apps and more.

Penpot is the first Open Source design and prototyping platform meant for cross-domain teams. Non dependent on operating systems, Penpot is web based and works with open web standards (SVG). For all and empowered by the community.

Penpot @ GitHub

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.