Auto Generate Database Diagram MySQL

Try MySQL Workbench, formerly DBDesigner 4:

http://dev.mysql.com/workbench/

This has a "Reverse Engineer Database" mode:

Database -> Reverse Engineer

I've recently started using https://github.com/schemaspy/schemaspy . It strikes me as having a good balance between usability and simplicity. (GraphViz now optional)

On a Mac, SQLEditor will do what you want.

This http://code.google.com/p/database-diagram/ will reverse engineer your database. Just do an export 'structure only' then paste the SQL into the tool.