Edit: Failed to include (my own) HILCC module. How embarassing.
Out of curiosity, I did a quick sampling of usage stats (according to Drupal.org) of different library-related Drupal modules. I left out some that didn't have usage information or are not hosted on D.O. and of course, didn't count many other modules useful to libraries (like Apache Solr, booking modules, etc.)
These stats also reflect only those sites that have module update tracking turned on; it'd be a good question to ask Drupal library service providers what's their best practice on that.
Usage statistics for EZProxy
Usage statistics for SOPAC
Usage statistics for The eXtensible Catalog (XC) Drupal Toolkit
Usage statistics for Millennium OPAC Integration
Usage statistics for Cite
Usage statistics for DigitalNZ API
Usage statistics for MARC
Usage statistics for OAI-PMH Module
Usage statistics for Library
Usage statistics for OAI2 for CCK
Usage statistics for Book Post
Usage statistics for HILCC Taxonomy autotag