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easy to use, completely free.
Violet is a UML editor with these benefits: Very easy to learn and use. Draws nice-looking diagrams. Completely free. Cross-platform. Violet is intended for developers, students, teachers, and authors who need to produce simple UML diagrams quickly
Design Patterns & Refactoring.
I'm SourceMaking. I will tell you a lot of stories about good software architecture and teach you how to create it with design patterns. I will guide you through anti-patterns, common pitfalls and mistakes that people make when they plan, create, and manage software projects. In the end, I will teach you how to smell a bad code and improve it with refactoring.
Gaphor is the simple modeling tool.
Gaphor is a UML and SysML 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.
UMLet is a free, open-source UML tool with a simple user interface: draw UML diagrams fast, build sequence and activity diagrams from plain text, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and create new, custom UML elements. UMLet runs stand-alone or as Eclipse plug-in on Windows, OS X and Linux.