Forum OpenACS Q&A: Response to Oracle vs. PostgreSQL
There are applications that require Oracle, but many don't. PostgreSQL is in use in many production sites and works great. PostgreSQL is much easier to install and administer than Oracle.
Read the article about PostgreSQL at phpbuilder.com and see for yourself if it's production worthy or not. openacs.org received 33,000 db-backed hits in one day when it appeared on Slashdot earlier this year, and PG/OpenACS/Linux/AOLserver handled it just fine.