Tool for Reverse Engineering of PostgreSQL Database on Linux (Ubuntu)
Have a look at at (the order of the items implies my personal choices)


DbSchema (not free, but not expensive: http://dbschema.com/)
SchemaSpy (http://schemaspy.sourceforge.net/)
Power*Architect (http://code.google.com/p/power-architect/) although development seems to have stopped) 


All of them work with PostgreSQL (and other DBMS)

(Note: I have nothing to do with DbSchema. I just think it's a pretty good tool)