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
UML Reverse Mapper is a utility that helps convert existing code structures into UML-like representations so developers can visualize and reason about their systems. It is aimed at teams that inherit codebases or work on legacy projects and need a fast way to understand classes, relationships, and dependencies. Instead of drawing UML by hand, you point the tool at the code and let it generate artifacts that resemble standard UML diagrams.
jUML is a UML diagram application capable of reverse engineering Java sourcecode to create relationship diagrams, sourcecode generation based on a user created UML diagram, and saving/restoring of jUML projects.