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.

Visual design studio for the development of javascript systems.

MakeHuman is the free and open source software to create realistic 3d humans.

Automated Drawing of UML Diagrams
UMLGraph allows the declarative specification and drawing of UML class and sequence diagrams. The current features are part of an ongoing effort aiming to provide support for all types UML diagrams. An IEEE Software article titled On the declarative specification of models explains the rationale behind this approach. The tehnology behind UMLGraph was used to draw many of the diagrams appearing in the award-winning books Code Quality: The Open Source Perspective (Addison Wesley, 2006) and Code Reading: The Open Source Perspective (Addison Wesley, 2003). In addition, the UMLGraphDoc doclet included in this distribution automatically adds UML diagrams to javadoc documentation.

UML is meant for greater things than simple description of data structures. Still the UML metamodel can support Schema design quite well, for wire-serializable types, persistence schema, and many other XML applications.

Papyrus is a dedicated tool for modelling within UML2. This open source tool is based on the Eclipse environment.