While doing some research on UML modelling for one of my current projects I ran across these tools for the job.
My search for tools began in my usual manner. Sourceforge.net Your one stop shop for free open source tools. The most feature rich and useable tool on there list for UML modellers was ArgoUML. It had one minor flaw though. I originally began the search because I needed to some sequence diagrams. ArgoUML, however, has not implemented that diagram type yet.
But wait. ArgoUML was the base for a commercial product: Poseidon for UML, and according to the website they had a free community edition. I downloaded the app installed and walla all the functionality of ArgoUML plus sequence diagrams. The community edition is free has no restrictions on save or export other than a watermark on your image exports. Some of the commercial additions to ArgoUML are not available but pretty much everything else is there.
I LOVE open source.
Update:
After trying the export to graphic option of Poseidon UML I was disappointed to learn that it was restricted to a small size range for the diagrams. So I guess I will be using ArgoUml after all.