Green, an Eclipse plugin, is an advanced yet simple UML class-diagram tool, developed primarily for student use. It does live round-tripping, so it's easy to move between diagram and code views. It's extensible: users can define their own relations
Recon-Calc is a source code feature location tool based on the Software Reconnaissance technique. It allows you to name features of your system and then, based on coverage profiles gathered, you can find where they are implemented in the source code.