You might want to check out
ArgoUML. It is what
Poseidon is based on. Personally, I am not a fan of either.
At work I use
Together Control Center, however I realize that not everybody has access to this level of UML tool.
At home I use
MagicDraw. It is not free but you can get an academic edition for $99. Since the actual assignment runs $250, I don't see that as a bad investment. It is a good tool.
[ December 24, 2002: Message edited by: Chris Mathews ]