I would suggest SQL Power Architect for the following reasons (from the official feature list):
- Accesses source databases via JDBC
- Connects to multiple source databases concurrently
- Compares data models & database structures and identifies discrepancies
- Drag-and-drop source tables & columns into the playpen
- Remembers the origin of each column
- Generates source-to-target visual Mapping Reports
- Forward/reverse engineers PostgreSQL, Oracle, MS SQL Server & more
- Saves source data structure snapshots in the project, so you can work remotely
- All project data is stored in an easy-to-parse XML format
- OLAP schema modeling: Cubes, Measures, Dimensions, Hierarchies & Levels
- GPL (version 3) licensed
...and a couple reasons of my own:
- runs on basically any platform
- has fairly nice keyboard support for fast model development
- relatively useful diagram auto-layout
