Institute for Snow and Avalanche Research (SLF), Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Davos, Switzerland
We have installed Debian on about 30 servers, running services like Samba, BIND, DHCP, FTP, mail (qmail), web, database (Oracle, MySQL and PostgreSQL) and backup (via rsync and Arkeia). We also have about 40 Debian desktop workstations, used primarily for development and office tools.
We use Debian because of its simplicity; and because it works that stable!