I found an excellent and freeware database modeling tool: Toad Data Modeler (TDM) 3.0 Free. It enables you to create ER diagrams, physical and logical modeling with verification, SQL script generation and much more. It has very simple UI that enables you quick model creation. It can target almost any database.
I also tried a previous version (version 2) and I was more than satisfied. However, keep in mind that this software is not a replacement for Visio. Unlike Visio which is a complete modeling tool, TDM encapsulates one functionality, and that is - database modeling.
If you want fully functional modeling tool you can purchase full version. If you are unsure which version is right for you, take a look at the feature comparison page.