ER diagram software

I've found MySQL Workbench to be most useful for creating "ER" diagrams.

Though the format is only MySQL Workbench specific I can take a diagram created in Workbench and export it to SQL and execute it against any number of SQL servers.

If you're working with MySQL this tool also doubles as an Administration and Querying tool as well providing the added benefit of integration between the three.

Dia

Dia is a GTK+ based diagram creation program for GNU/Linux, Unix and Windows released under the GPL license. For more information visit the Dia project page

Umbrello can also create ER-Diagrams besides most other UML diagrams. In addition it allows you to export the Diagrams in form of code (i.e. in this case SQL).