ArgoUML is a free, open-source UML diagramming tool that is gaining more popularity of late. It is maintained at tigris.org
Do you want these diagrams for internal documentation or external? If internal, I would suggest that you simply draw a nice diagram on the board and take a snapshot using a digital camera.
You really want the content of the diagram, not how pretty it looks. I have seen lots of person-hours spent on battling with an UML tool only to throw away (or shelf) the design after a few months. For such situations, a picture speaks a lot more than a 1000
Rose diagrams!!!