Visually designing a database structure

I use the aptly named Database Design Tool. It's extremely simple and unfortunatly it's developed any more, however. It's the best tool I've come across that is free and at the end of designing your tables, it generates the T-SQL for you. It's also language independent.

Open System Architect has some potential. Its very similar to Visio.

I'm a big fan of ARGO UML from Tigris.org. Draws nice pictures using standard UML notation. It does some code generation, but mostly Java classes, which isn't SQL DDL, so that may not be close enough to what you want to do.

You can look at the Data Modeling Tools list and see if anything there is better than Argo UML. Many of the items on this list are free or cheap.

Also, if you're using Eclipse or NetBeans, there are many design plug-ins, some of which may have the features you're looking for.

Well on the PC you can use MS Visio to produce a DB Entity diagram.

It will even reverse engineer one from an existing Database.

A pain to set-up the first time you use it, but quite handy thereafter.