WebObjects/Database Compatibility and Comparisons/Postgresql

Yes, PostgreSQL does work with WebObjects. The best place to start is the Project WONDER PostgresqlPlugin Page.

If you're on a Mac, Apple has an article on installing PostgreSQL on Mac OS X that is pretty straightforward.

If you're interested in a nice graphical interface to managing your PostgreSQL installation, you may want to checkout out Navicat PostgreSQL from Navicat.