OPAC is a newish module for Drupal 7, that
enables libraries to integrate their catalog into Drupal allowing importing records, make advanced searches with faceted results, circulation task etc … This module is intended to work with any ILS by using connectors. Everyone can make its own by creating a php file.
Read the full description at drupal.org/project/opac
It was developed by BibLibre, a French company doing Open Source for libraries. Their blog entry gives more background information.