BiblioteQ is a complete and thorough library management tool. It sports a Qt 4.x user interface and provides support for both MySQL and PostgreSQL. The Z39.50 protocol is used for information retrieval from the Library of Congress.
As for database support, BiblioteQ users can connect to either MySQL or PostgreSQL database engines. The application is being designed in the hopes of providing common functionality across FreeBSD, Linux, Mac OS X, Solaris, and Windows.