Here in the Universidad de Buenos Aires' datacenter, we have hundreds of Debian servers which manage Infrastructure facilities such as DHCP, VoIP, LDAP, SMTP, proxies, and firewalls. We also run services such as MySQL, PostgreSQL, Oracle, Apache2, Tomcat, Glassfish, and Moodle. We happily also host a Debian mirror. Debian was chosen due to it's stability and tools availability.
We chose Debian because it is stable, simple and fast to install and to use, and because it is Free Software.