Alll our servers run the most stable and secure version of Linux: Debian.
Debian was founded in 1993 by Ian Murdock, then a student at Purdue University, who wrote the Debian Manifesto which called for the creation of a Linux distribution to be maintained in an open manner, in the spirit of Linux and GNU. He chose the name by combining the first name of his then-girlfriend (now wife) Debra with his own first name "Ian", forming the portmanteau "Debian", pronounced as debian (deb-e'-en).
On top of Debian we use other Open Source products:
Apache Web Server
Exim Mail Server
SpamAssassin
MySQL database
PHP