OPACIAL is an open source Web 2.0 Online Public Access Catalogue (OPAC). It is based on PHP-Mysql (requires Yaz and GD). The users can isnert their own social tags/opinions and have a faceted search/browse interface (also using social tags).
PLEIAS is an open-source Inter-library Loan System (ILL) written in PHP-Mysql. Some of its major features are: custom workflow and account types, customer and supplier financial monitoring, group accounts, customer web interface for order tracking.