Although it is not specifically designed for servers, Debian GNU/Linux is a popular choice for system administrators because of its stability, security, and long stable release cycle. Debian has also been around for a very long time (19 years) and is regarded as one of the most reliable operating systems in terms of staying true to free and open source software principles.
Debian’s vast software repository includes most of the applications a sysadmin would need to get a server running, and it also allows the administrator to easily fine tune each application’s configuration. While desktop users may criticize the project for its long stable release cycle, it may be beneficial to server administrators who need long-term stability for their servers.
Debian relies on the APT package management system, which is regarded as one of the easiest tools for Linux application installation. As such, Debian has become one of the most popular parent distributions, having spawned a multitude of derivatives, including the world-famous Ubuntu.