The Department of Informatics at the University of Zurich uses almost exclusively Debian/GNU Linux on their server infrastructure. Currently we run Debian on around 75 physical and 70 virtual machines (KVM). Figures which are constantly growing.
Services provided on these machines include DNS, DHCP, LDAP, SMTP, IMAP, webmail, mailing lists, CalDAV, Samba, NFS, backup, Subversion and Git repositories, web proxy, Drupal and Joomla websites, MediaWiki, Tomcat, PostgreSQL and MySQL, configuration management and inventory.
The most important reasons to us for using Debian is its stability, security and upgrade-ability. Furthermore, we also like its flexibility when it comes to mixing in packages from testing or unstable to systems running Debian stable. This enables us to profit both from the stability of Debian (based on the conservativeness of Debian stable) and to be up to the state of the art for exactly those packages where it is required. Additionally, Debian is among the most complete distributions, i.e. most of the software we need is already packaged for Debian.