Free in this month's Computers in Libraries (I get paid to write a column in same, but had no idea about this until now) is this piece by Mike Beccaria and Dan Scott :
"Fac-Back-OPAC  is a faceted backÂ up OPAC. This advanced catalog offers features that compare favorably with the traditional catalogs for todayâ€™s library systems. Fac-Back-OPAC represents the convergence of two prominent trends in library tools: the decoupling of discovery tools from the traditional integrated library system and the use of readily available open source components to rapidly produce leading-edge technology for meeting patron and library needs. Built on code that was originally developed by Casey Durfee in February 2007, Fac-Back-OPAC is available for no cost under an open source license to any library that wants to offer an advanced search interface or a backup catalog for its patrons."