UML Designer!

PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams. The generator can be run anywhere within JVM and integrated with various application such as wiki, text editor, IDE, programming language, documentation generator, and others. Mostly based on Graphviz. Can render various diagram syntax such as PlantUML, Dot, and Ditaa.

UMLet is an open-source UML tool with a simple user interface: draw UML diagrams fast, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and create new, custom UML elements.

Visual Paradigm for UML (VP-UML) is a UML design tool and UML CASE tool designed to aid software development. VP-UML supports key modeling standards such as Unified Modeling Language (UML) 2.4, SysML, ERD, DFD, BPMN 2.0, ArchiMate 2.0, etc. It supports software development teams in requirements capturing, software planning (use case analysis), code engineering, class modeling, data modeling, etc.

Umbrello UML Modeller is a Unified Modelling Language diagram programme for KDE. UML allows you to create diagrams of software and other systems in a standard format.

Enterprise Architect is a collaborative modeling, design and management platform based on UML and related standards. Agile, intuitive and extensible with fully integrated, powerful domain specific high-end features at a fraction of the cost of many competitors. An enterprise wide solution for visualizing, analyzing, modeling, testing and maintaining a wide range of systems, software, processes and architectures. Team based, with a proven track record and close to 200,000 users worldwide.

Graphical modelling open source UML2 tool based on Eclipse environment.

WhiteStarUML is a fork of StarUML with an intent to revive its Delphi code base by updating code to recent Delphi editions, reducing dependence on third party components and fixing bugs and adding new features.

BOUML is a free UML 2 tool box (under development) allowing you to specify and generate code in C++, Java, Idl, Php and Python.

Whether you are a Business Architect, System Architect, Developer, Analyst or Designer, Modeliosoft meets your needs through its high quality solutions for modeling, and report, architecture or code generation. Modeliosoft Solutions integrate UML, BPMN, requirements analysis and many other modeling extensions and enable you to configure a bespoke solution for your needs.