Penpot

Penpot is an open-source design and prototyping tool that works in the browser, an alternative to Figma. You design screens and user interfaces, create clickable prototypes and work together as a team on the same file. The professional cloud plan is free with no limits; the paid plans only add team management and hosting. You can also install it on your own server. It suits engineers designing an interface for software, a machine or a control panel.