Software For Diagramming Database Relations?

etrnl asks: "I'm designing several websites with databases, and even designing a message board that supports multiple RDBMSes. What I'm looking for is a tool under Unix that I can use to diagram the relationships of different databases. Right now I'm using myODBC, myAccess, and Access 2k but there's got to be something better in the Open Source community (outside of using dia by hand...). I'm looking for support for mySQL, PostgreSQL, and Oracle if possible."