Is there a good free conceptual modeling tool for EER modeling? It should support specialization, generalizaton, union, etc. I came across MySQL Workbench but it seemed to belong to the physical modeling stage(examples that I saw included 'VARCHAR' or 'INTEGER' specifications for attributes and it deals with tables rather than entities).
Open System Architect is brilliant.
SQL Power Architect .
It's community edition is free, and does a really great job helping you build your ERDs.
It works on Windows, Linux, and OSX.