Do you hate starting on a new project and having to try to figure out someone else's idea of a database? Or are you in QA and the developers expect you to understand all the relationships in their schema? If so then this tool's for you.
SchemaSpy is a Java-based tool (requires Java 1.4 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. The browsing through relationships can occur though HTML links and/or though the graphical representation of the relationships. It's also designed to help resolve the obtuse errors that a database sometimes gives related to failures due to constraints.