You can get a good list of Open source UML tools @ java-source.net. The ones recommended on personal experiences from my friends are
- Drawing Class Diagrams and Sequence diagrams
- Reverse engineering classes from Java code
- Copy Paste of diagrams to MS Word
- Plugin to the Eclipse IDE
- Drawing Sequence diagrams and Class Diagrams
- Class Diagram reverse engineering
- But Sequence diagram reverse engineering feature is available only with enterprise edition.