BAREFEET  I tested SQLEditor with an SQLite database. It imported the table structures fine, using the appropriate driver. After playing with it a bit, I realize that SQLEditor only shows relationships between tables that are set up as a "foreign key" within the tables. SQLite (and I think others) don't have a foreign key parameter for fields within tables. I set up relationships in SQLite within views. Unfortunately, SQLEditor does not appear to show views or the relationships tat they contain. So it seems of little use to show or edit the structure of my SQLite databases.  (Version 1.4b19)

KIDDAILEY  Just thought I'd post an updated comment because my original one below is a bit outdated.

I've been using this for years now, and it is by far the best ER diagramming tool I've used on OS X or Windows.

Everything is straight-forward. You don't have to hover over buttons to figure out what they do and most of the work takes place in the canvas area (and optionally, the inspector) keeping you from having to constantly return to a toolbar or menu.

The layout tools are simple. Color, notes and canvas areas make it easy to organize and group segments, types and blocks of entities around the work area.

Printed output is fantastic and you can turn on page-breaks as well, enabling you to move objects to fit on individual sheets just as you want.

Import and export tools are awesome, and being able to see the raw query as you work is handy.

I use it for both MS SQL and MySQL. Though it doesn't (yet?) come configured to diagram for MS SQL Server, it is fairly trivial to get it configured for it in terms of diagramming. For MySQL and a few others, it's perfect out of the box.

The developer is responsive and the software is actively maintained.

For any serious DB architect on the Mac, this is worth every penny.  (Version 1.4b19)

SILVERDR  I have to admit that the app is quickly entering maturity stage and may well be one of the quite popular among database design enabled OS X users. The recent version even managed to properly read-in and parse my hand-made DDL with lots of nasty things inside ;-) I still have some doubts concerning the data types, which could be more aligned to what the (chosen?) RDBMS supports but this is in no way a show-stopper. The author is supportive, reacts to bug reports and suggestions quickly and efficiently. Price, while not cheap is quite moderate and IMHO not too high.  (Version 1.3)

JOHN  Excellent, excellent, excellent! This great ER Diagramming tool just keeps getting better with every release.

It's got the basic functions for creating simple diagrams that can be exported to a few different SQL formats.

It doesn't easily allow you to specify field lengths, and it's got a few minor bugs, but otherwise it's a great tool.  (Version 1.0CB11a)