UML diagrams using IntelliJ
IntelliJ IDEA (Ultimate Edition) supports UML class diagrams.
Some documentation is available on the Intellij WebHelp site in the Diagram Reference section.
However, users might have specific questions not sufficiently covered in the Web Help. Here are some links to Stack Overflow questions and answers:
- How to use “Show Dependencies” (see icon):  
- How to add classes to a diagram
- How to do “Forward engineering” (i.e. add diagram elements to produce code)
- How to use keyboard shortcuts in the UML diagram editor
To illustrate a possible result, I used the “Export to File” feature (look for the icon) to save this example diagram in SVG format.
Note for users of Microsoft Internet Explorer: For IE 8 or older you need to install Adobe’s SVG Viewer plugin, whereas IE 9 and newer allegedly have basic SVG support “out of the box”.