Stefano Demiliani wrote about the open-source tool DBDesinger in his weblog.

This database tool is designed for MySql but should support Oracle and MS SQL as well. You'll find a comment by Shannon J Hager on Paschal L's weblog saying that script generation will fail for MS SQL 2000. However, have a look at this tool - it's best free tool I've ever seen.

Feature list taken from DBDesinger website:

DBDesigner 4 - Features

DBDesigner 4 provides powerful features to create a visual model of any database.

Starting from a reverse engineering engine to automatically retrieve a model from existing databases, extensive modeling tools and editors to a syncronisation function which will apply model changes automatically to the underlying database - it is all part of DBDesigner 4.


Design Mode vs. Query Mode

DBDesigner 4 supports two switchable userinterfaces. The Design Mode is used to create and maintain the visual databases model. The Query Mode is used to work with table data and build complex SQL query statements for the use in PHP, Kylix or another programming language.


Plugins and Open Source

The models created in DBDesigner 4 are stored in XML. They can be modified by third party plugins which are direcly launched from within DBDesigner 4 and other third party products.
Because DBDesigner 4 is an Open Source project it is easy for programmes to develop new plugins or expand DBDesigner 4 to adapt specific needs.


List of features

• Available on Linux / MS Windows
• User Interfaced based on industry standard layouting software
  • Canvas navigation similar to Adobe Illustrator® and Photoshop®
  • Palettes (docked / floating) including "Bird Eyes view"
  • Available objects include tables, realtions, labels, regions, images
  • Extensive Drag'n'drop support
  • Extensive Popup-Menu support
  • Advanced Editors
  • UNLIMITED Undo-/Redo- functions
  • Copy-/Cut-/Paste clipboard functions (XML, DDL)
  • Align functions
  
  
• Design Mode / Query Mode
• Reverse engineering MySQL, Oracle, MSSQL and any ODBC databases
• Userdefined schema-generation
• Model-To-Database syncronisation
• Index support
• Automatic foreign key placement
• Weak entity support
• Standard Inserts storing and sync
• Full documentation capabilities
• Advanced model printing
• Output as image
• All MySQL datatypes with all options
• User defined datatypes
• Database storage, ability to save model within database
• Network-/Multiuser access through database storage
• Version control*
• SQL Query Builder
• SQL Command History
• SQL Command storage within model
• Plugin interface

* Version control only available when using database storage