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.
Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.
Pencil Project vs Moqups opinions
Wireflow is free, online, real-time collaborative, open source tool for creating beautiful user flow prototypes.
Diagramo is an online flowchart software
- diagrams (create, edit and share)
- organization charts (create, edit and share)
- network diagrams (create, edit and share)
- graphs (create, edit and share).
ProcessMaker is a cost effective and easy to use open source business process management (BPM) or workflow software application. Workflow software such as ProcessMaker can assist organizations of any size with designing, automating and deploying business processes or workflows...
Napkee makes your Balsamiq Mockups come alive. Import a BMML and export to web or flex code in a few clicks.Discontinued
The program seems to be no longer updated. Last version, 2.0 alpha 3, released on October 30, 2013, can be still downloaded from the official website.