Have found an open-source and freeware tool for making UML diagrams.
It's called StarUML. You can download it from the StarUML.com website.
I used it for sometime and found it to be rather stable and usable. It most definitely is not crippleware. Can't say the same about Poseidon or the other community-edition tools like Visual Paradigm or UMLStudio.
StarUML is definitely recommended, though it is not without its share of glitches.