Firefly UML
A free, easy-to-use and user-friendly Java-based UML editor.
... how to deal with the other diagram types, because they all follow the exactly same user interaction patterns.
Besides being user-friendly, Firefly also offers powerful features which other free editors usually do not. For example, you can import your existing Java project into Firefly and generate nice-looking class diagrams within a matter of minutes. You can also go the other way around and export your model as skeleton Java code.