free database modelling software
There are several choices:


SQL Power Architect (download without registration from Google Code). This is open source, but it seems there is no further development.
Oracle's Data Modeler (but it only supports Oracle, SQL Server and DB2. PostgreSQL for example is not supported)
A free version of TOAD Data Modeler - although that has some serious limitations on the size of the model
Open ModelSphere a general purpose modeler (including UML) but can also be used for ER modelling