As a developer, DBA and consultant, I know what a nightmare it is to walk into a site without being able to get your head around the data structures in place. I've started to evaluate SQLSpec, and so far it looks pretty good.

From their website:

Do you wish you had comprehensive and uniform database documentation for all your applications, regardless of platform?

Are you a developer or DBA that just inherited a huge, complex application with no supporting database documentation, and don't know where to start learning how it is put together?

Are you an independent contractor working with databases? Your clients will LOVE you when you provide comprehensive database documentation for the applications you build.

SqlSpec is a database documentation tool that will generate easy-to-read database documentation for any of the following platforms:

    * SQL Server 2000/2005/2008
    * Analysis Server 2005
    * Oracle 9i and above
    * DB2 8.2 and above
    * Sybase ASE 12.0 and above
    * Informix IDS 10.0 and above
    * PostgreSQL 8.0 and above
    * MySQL 5.0 and above
    * Access 97 and above
    * VistaDB 3.0 and above
    * ENEA Polyhedra 7.0 and above
    * SQL Anywhere (beta)

Get it here.