Archi® is a free, open source, cross-platform tool and editor to create ArchiMate models.
The Archi® modelling tool is targeted toward all levels of Enterprise Architects and Modellers. It provides a low cost to entry solution to users who may be making their first steps in the ArchiMate modelling language, or who are looking for a free, cross-platform ArchiMate modelling tool for their company or institution and wish to engage with the language within a TOGAF® or other Enterprise Architecture framework.
ArchiMate® is an open and independent Enterprise Architecture modelling language that supports the description, analysis and visualization of architecture within and across business domains. ArchiMate is one of the open standards hosted by The Open Group and is fully aligned with TOGAF®.
Archi website:
All developer resources are here:
https://github.com/archimatetool/archi/wiki/Developer-Documentation
Please see How can I contribute code to Archi?