Linux
According to Wikipedia:
Linux (also known as GNU/Linux) is a Unix-like computer operating system. It is one of the most prominent examples of open source development and free software; unlike proprietary operating systems such as Windows or Mac OS, all of its underlying source code is available to the general public for anyone to use, modify, and redistribute freely.
Initially, Linux was primarily developed and used by individual enthusiasts on personal computers. Since then, Linux has gained the support of major corporations such as IBM, Sun Microsystems, Hewlett-Packard, and Novell for use in servers and is gaining popularity in the desktop market. It is used in systems ranging from supercomputers to mobile phones. Proponents and analysts attribute its success to its security, reliability, low cost, and freedom from vendor lock-in.
We at Caseproof are committed to Linux and Open-Source solutions for our customers. The “security, reliability, low cost, and freedom from vendor lock-in” enable us to meet the needs of your business without the prohibitive costs of other software applications. In the Linux world there are several different flavors (or distros), the most popular being Red Hat, SuSE, Debian and Ubuntu. We are partial to Ubuntu because of its elegance and similarity to Debian, but we also use Red Hat Enterprise and SuSE Linux. All of our Shared and Mail Servers run on Ubuntu Linux, but our dedicated servers run various distributions depending on customer needs.