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
http://drupal.org/project/usage/ezproxy
Usage statistics for SOPAC
http://drupal.org/project/usage/sopac
Usage statistics for The eXtensible Catalog (XC) Drupal Toolkit
http://drupal.org/project/xc
Usage statistics for Millennium OPAC Integration
http://drupal.org/project/millennium
Usage statistics for Cite
http://drupal.org/project/cite
Usage statistics for DigitalNZ API
http://drupal.org/project/digitalnz
Usage statistics for MARC
http://drupal.org/project/marc
Usage statistics for OAI-PMH Module
http://drupal.org/project/oai2
Usage statistics for Library
http://drupal.org/project/library
Usage statistics for OAI2 for CCK
http://drupal.org/project/oai2forcck
Usage statistics for Book Post
http://drupal.org/project/bookpost
Usage statistics for HILCC Taxonomy autotag
http://drupal.org/project/hilcc