yEd is a very powerful graph editor that is written entirely in the Java programming language. It can be used to quickly and effectively generate drawings and to apply automatic layouts to a range of different diagrams and networks.
UMLGraph allows the declarative specification and drawing of UML class and sequence diagrams. The current features are part of an ongoing effort aiming to provide support for all types UML diagrams. An IEEE Software article titled On the declarative specification of models explains the rationale behind this approach. The tehnology behind UMLGraph was used to draw many of the diagrams appearing in the books Code Quality: The Open Source Perspective (Addison Wesley, 2006) and Code Reading: The Open Source Perspective (Addison Wesley, 2003). In addition, the UMLGraphDoc doclet included in this distribution automatically adds UML diagrams to javadoc documentation.