All the client and server systems in our computer department's various computer labs run Debian GNU/Linux. We have 60 workstations, 5 servers, and 20 Remote Boot thin clients. We've been using Debian for the past two years. It is of course the most stable OS we ever had.
Our servers run software such as apache, proftpd, wu-imap, exim, squirrelmail, squid, bind, sshd and a lot more. All our workstations use icewm, kde or gnome.