protosquared is a "data server prototype". It supports several data sources, such as XML and SQL databases and the communication protocol is highly configurable too.
Postgresql AutoDoc is a utility which will run through PostgreSQL system tables and returns HTML, DOT, and 2 styles of XML which describes the database.
Glom is a GUI that allows you to design MySQL table definitions and the relationships between them. It also allows you to edit and search the data in those tables.
mysqldiff is a Perl script which compares the data structures (i.e. table definitions) of two MySQL databases and returns the differences as a sequence of MySQL commands suitable for piping into mysql which will transform the structure of the first database to be identical to that of the second.