Open ModelSphere

Open ModelSphere is a great modeling tool for software projects of any size. You can design Data Models, Business Process Models (BPMs) and UML Models.

It's Java based, so you will need to install the latest JRE in order to use it.
The user interface is not so colorful and good-looking as by other products like MySQL Workbench, and its operation is not so intuitive like the rest.
But he bigger strength of this product is that it's freeware, but you have no limitation at all to print, export or reverse-engineer your models.
Yes, that's correct: if you install the JDBC driver for your favorite DBMS (MySQL in my case), you can easily reverse (and even forward!) your database (build your model based on an existing database and vice-versa).
This program has very few drawbacks taking into account its numerous advantages, for example: its installer is designed to work on Windows systems only. But the boost of productivity and project quality that you obtain using this software makes it a must-have for developers and system analysts.