Gaphor

Program: Gaphor

License: Open Source/Apache License Version 2.0

Description: Gaphor is a UML, SysML, RAAML, and C4 modeling application written in Python. It is designed to be easy to use, while still being powerful. Gaphor implements a fully-compliant UML 2 data model, so it is much more than a picture drawing tool. You can use Gaphor to quickly visualize different aspects of a system as well as create complete, highly complex models.

Website: https://gaphor.org/
Website (Development): https://github.com/gaphor/gaphor

Other: It has a Portable version.