Debian is an OS that uses the Linux kernel. Though it is usually set up on desktops, it's being used on a growing number of web servers as well. The reason is that it is very stable, so a server running it won't have to be restarted for months. Debian is also regarded as being one of the most protected Operating Systems you can get, because security patches are released within days after an issue is discovered. This is possible since thousands of well-experienced developers work for the Debian project on a voluntary basis and not only do they work on the OS itself, but they also provide 24/7 support via special e-mail lists and respond within 20 min to any kind of question. Thus, you'll enjoy top-level support directly from the developers who know their own platform in depth, and not from a third-party support team. Debian is also one of the Linux distributions with the highest number of packages - more than 30,000. As the Operating System is completely free to use, you can modify both the core as well as all the packages in any way you see fit. Furthermore, you won't be required to pay any kind of license fees.