PenPot is a free open source design and prototyping platform. It works through a web browser and uses open standards, so it can be used collaboratively and on any OS.
You can follow at:
PenPot is now in beta testing, find out more at https://penpot.app
(PenPot used to be called Uxbox)
A newer server operated by the Mastodon gGmbH non-profit