DIVA - is a viewer of database physical structure, it can show you any section of DB in several usable forms (2D, 2.5D, 3D). In this way user can easily understand dependency between tables.
SchemaSpy is a Java-based tool (requires Java 5 or higher) that analyzes the metadata of a schema in a database and generates a visual representation of it in a browser-displayable format. It lets you click through the hierarchy of database tables via child and parent table relationships as represented by both HTML links and entity-relationship diagrams. It's also designed to help resolve the obtuse errors that a database sometimes gives related to failures due to constraints.
DB-SVG is a java web application that generates SVG data model diagrams for any database with a JDBC driver. Database connections are configured via an XML file. The interactive interface allows tables to be dragged around and their positions saved. It is designed to make internal database coordination and documentation easy and accessible. See the demo to understand how it works.