TextUML Toolkit is an open-source IDE for UML

TextUML Toolkit is an open-source IDE for UML that lets you create models at the same speed you write code. By adopting a textual notation, the TextUML Toolkit provides benefits you will not get elsewhere:

• increased modeling productivity
• the features you like in your favorite IDE: instant validation, syntax highlighting, outline view, textual comparison
• live graphical visualization of your model as class diagrams
• compatible with any tools that support Eclipse UML2 models

Will you only believe it if you see it? Download and install the TextUML Toolkit (it is free!) and run through an easy-to-follow tutorial. Questions and problem reports can be posted on the user forum and issue tracker, respectively.