Pencil is a free tool for drawing diagrams and prototyping user interfaces. Designers and developers use it to make sketches and mockups. The current version is a desktop application built on Electron, and documents are stored in its own file format. A new version 3 has been released which, according to the source readme, replaces the older Mozilla XULRunner platform.
Pencil is developed by Evolus, a Vietnamese limited liability company that also offers custom software and medical management software. The source code is on GitHub under the GPL-2.0 licence; the latest code changes date from June 2026, while the latest tagged release, 3.1.1, dates from 2023. Because it is a local app, your files stay on your own device and no cloud account is needed. For a European organisation this means control over the files, but the maintainer is based outside the EU.