Debian

Debian, also known as Debian GNU/Linux, is a Linux distribution composed of free and open-source software, developed by the community-supported Debian Project, which was established by Ian Murdock on August 16, 1993. The first version of Debian was released on September 15, 1993, and its first stable version was released on June 17, 1996. It is one of the oldest operating systems based on the Linux kernel.

Many alternative Linux distributions are based on Debian, including Ubuntu, Linux Mint, Tails, and many others. 

I do not currently use Debian for any of my servers or desktops, however I am planning to evaluate it as a server OS alternative in the near future. The primary reason is its renowned stability and efficient use of resources. Whether those are better than Ubuntu Server or not, we will see.

Debian Cinnamon Desktop

Installation

Installation instructions for the various Debian supported architectures and flavors can be found on the official Debian website.