SchemaSpy analyzes database metadata to reverse engineer dynamic Entity Relationship (ER) diagrams. It works with just about any JDBC-compliant database (Oracle/MySQL/DB2/SQL Server/PostgreSQL/Sybase/etc.) and can identify Ruby on Rails as well as other implied relationships.
Mogwai ERDesignerNG is a Java based 2D & 3D Entity Relationship design & modeling (ERD) tool for Oracle, MSSQL, Postgres and MySQL. It includes database change and dictionary management, SQL generation, reverse and forward engineering plus reports and data browsing.