UMLet – Free UML tool for fast UML diagramming

UMLet is a free and open-source lightweight UML drawing tool (rather than a modelling tool with underlying dictionary or directory of reusable design objects) for Windows, Mac and Linux platforms, written in Java. It is powerful and easy to use, can be used as a standalone app, can also be used as a plug-in for Eclipse, and even provides a browser-based online version.