SchemaSpy is a neat tool to produce visual diagrams for most relational databases.
Here's how to use it to generate schema relationship diagrams for PostgreSQL databases:
Download the jar file from here (the current version is v6.1.0)
Get the PostgreSQL JDBC driver (unless your installed version of java is really old, use the latest JDBC4 jar file)
Run the command against an existing database. For most databases, the schema (-s option) we are interested in is the public one: