Bayour.com, Gothenburg, Sweden

I'm using Debian GNU/Linux on one primary storage server (hexacore AMD Phenom II, 16GB memory) which exports disks: 16TB on ZFS via Samba, NFS, iSCSI, AirVideoServer, AFP and DAAP. A firewall, running from a USB stick uses a heavily modified Debian Live with a dual 1Tb network card for broadband and internal network connection. The firewall is also responsible for internal NTP, DHCP and DNS services, to simplify setting up a new machine. Another server (AMD Athlon II, 8GB memory) hosts all my virtual machines, running directly from iSCSI on VirtualBox.

There is currently about 11 virtual machines running all the time, most of them on Debian GNU/Linux, used for development, testing, VoIP switching (Astrix) etc. User authentication and authorization is done using OpenLDAP and MIT Kerberos V across the network. All machines are headless, but a VNC server is running on the storage server just in case.

I chose Debian GNU/Linux because of simplicity and extensive software repository. You just can't beat over 6000 packages available! No matter the need, chances of it being packaged and ready to run, after a simple apt-get install package, is huge.