For free UML, try AguUML (http://www.tigris.org). There's also a community Edition of the Poseidon UML tool (http://www.gentleware.de), which is based on the ArgoUML source, but is more polished in many ways (though also crippled in a few). If you actually felt like buying a UML tool, theirs is much more reasonably-priced than most.
For the strict DBMS/ORM type of design, check out DBDesigner. It's not a UML tool, but it allows you to graphically layout your schema. You can then use MiddleGen to build the persistency info for frameworks such as Hibernate.
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