Which is the Best Open Source UML Modeler?

Here are the best open source UML modeller  (designing tools) and their websites for further reading.

1. BOUML - an UML tool box
BOUML is a free UML 2 tool box allowing you to specify and generate code in C++, Java, Idl, Php and Python

2. Modelio Open Source Community
Modelio   is an open source modeling environment. Modelio delivers a   broad-focused range of standards-based functionalists for software   developers, analysts, designers, business architects and system   architects. (GPL)

3. Papyrus
Papyrus   is aiming at providing an integrated and user-consumable environment   for editing any kind of EMF model and particularly supporting UML and   related modeling languages such as SysML and MARTE. (EPL)

4. Umbrello - The UML Modeller
Umbrello UML Modeller is a Unified Modelling Language diagram program for KDE. (GPL)

5. UML Designer 
UML   Designer provides a set of common diagrams to work with UML 2.5  models.  The intent is to provide an easy way to make the transition  from UML to  domain specific modeling. (EPL)

6. Umple: Merging Modeling with Programming
Umple   is technology for Model-Oriented Programming. It adds abstractions  such  as UML associations, attributes, and state machines to  object-oriented  programming languages such as Java, PHP and Ruby. (MIT)
Best Regards

Useful UML Resources

• OMG UML Specification
• Agile UML modeling software
• UML tutorials