We use Debian on four rack servers (IBM X3650M3). Each of server is virtualized using OpenVZ/KVM with Debian as the host operating system and the guests are also running Debian. Each instance is used to run different kinds of services like web server, mail server, firewall, proxy, DNS server, DHCP server, SVN server, and many other services.
We chose Debian for its great stability, support and the best democracy amongst the communities of developers. Thanks Debian!