There were days when UML tools were expensive and only few commercial ones were available. But, now the market is flooded with several commercial and opensource uml tools. While choice is good, but too much choice is also bad. It has become difficult to evaluate and select the best one.
In this article I present my handpicked collection of best open source UML tools.

1. Papyrus UML

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

2. Taylor

Taylor MDA is a specialized UML modeling tool based on Eclipse. It uses convention-based techniques to generate the maximum code from streamlined UML models. Templates are included for generating JEE applications based on JPA/EJB3 and JSF/Seam/Facelets.

3. TinyUML

TinyUML is a free software tool for easy and quick creation of UML 2 diagrams. It is implemented for the Java platform. At least Java SE 6 is required.

4. UMLet

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.
UMLet runs stand-alone or as Eclipse plug-in on Windows, OS X and Linux.

5. Frame UML

Frame UML is a free UML tool that supports UML 2.xx. It can run on Windows(2000/XP/Vista) and generate source code /CHM from model, or parse source code into model by JavaScript. It supports 12 kinds of diagram except object diagram. It is very nice,simple, and easy to use.

6. Umbrello UML Modeller

Umbrello UML Modeler is a UML diagram tool that can support you in the software development process. Especially during the analysis and design phases of this process, Umbrello UML Modeler will help you to get a high quality product. UML can also be used to document your software designs to help you and your fellow developers.

7. Eclipse UML2 Tools

UML2 Tools is a set of GMF-based editors for viewing and editing UML models; it is focused on (eventual) automatic generation of editors for all UML diagram types.
The diagrams that will be initially supported (as part of the first release) include class, component, state machine, and activity diagrams.

8. BOUML

BOUML is a free UML 2 tool box that allows you to specify and generate code in C++, Java, Idl, Php and Python. It runs under Unix/Linux/Solaris, MacOS X(Power PC and Intel) and Windows. It is also very fast and doesn?t require much memory to manage several thousands of classes.

9. ArgoUML

ArgoUML is the leading open source UML modeling tool and includes support for all standard UML 1.4 diagrams. It runs on any Java platform and is available in ten languages.

10. Acceleo

Acceleo is a code generator transforming models into code (MDA approach).

Acceleo is easy to use. It provides “off the shelf” generators (JEE, .Net, Php…) and template editors for Eclipse.

11. Netbeans UML Plugin

The NetBeans UML plugins presently supports the following UML diagrams: Activity diagram, Class diagram, Sequence diagram, State diagram, and Use Case diagram. You can align diagrams visually in the Diagram Editor, Drag and drop roles, patterns and classes from simplified Modeling Palette and add relationships from the Context Palette.

12. StarUML

StarUML is an open source project to develop fast, flexible, extensible, featureful, and freely-available UML/MDA platform running on Win32 platform. The goal of the StarUML project is to build a software modeling tool and also platform that is a compelling replacement of commercial UML tools such as Rational Rose, Together and so on.