What FLOSS DB design tool for Linux would you recommend?

For MySQL there is MySQL Workbench:

MySQL Workbench is a cross-platform, visual database design tool developed by MySQL. It is the highly anticipated successor application of the DBDesigner4 project. MySQL Workbench will be available as a native GUI tool on Window, Linux and OS X.

You can have a look at WWW SQL designer

WWW SQL Designer allows users to create database designs, which can be saved/loaded and exported to SQL scripts. Various databases and languages are supported. Ability to import existing database design.

This tool allows you to draw and create database schemas (E-R diagrams) directly in browser, without the need for any external programs (flash). You only need JavaScript enabled. The Designer works perfectly in Mozillas (Firefox, Seamonkey), Internet Explorers (6, 7, 8), Safari and Operas. Konqueror works, but the experience is limited.

Code license: New BSD License

Take a look at DBDesigner 4. Its a MySQL database design tool that also supports other database products like SQL Server, Oracle, SQL Lite and ODBC.

Its free, opensource (GPL), available for Linux/GNOME.

Some interesting features:

• If you are using a database not directly supported, you can create your own data type. Right click in the data type window and select Create New Datatype.
• You can connect to an existing database and it will import your model.