CIE ADEMUR, Spain
We are using Debian GNU/Linux since 2006 mainly as Samba domain controller and heavy loaded Samba file server, among another services like ssh for remote administration, OpenVPN server, dynamic DNS, and NTP server. After some Intel x86 boxes, our current server is a HP C3750 workstation, inside it has a PA-RISC processor, we use GNU/Linux RAID 1 features to have data redundancy.
We chose Debian GNU/Linux due to its stability, multi-architecture support, amount of packages, amount of documentation and its independence from market manipulations.