Computer Engineering Department, Sharif University of Technology, Iran
We currently have three servers that are running Debian, and the only non-Debian OS used is an OpenBSD. Some of the packages we use are: openldap+bdb, samba, postfix, mailman, courier-[imap,pop3], squirrelmail, amavis-new, clamav, syslog-ng, bind9, freeradius, mysql, apache2, vsftpd.
Our decision to use Debian is based on its package management system (APT), ease of maintenance, security patches and stability.