Debian is an OS which uses the Linux kernel. Though it is usually installed on desktop computers, it is being used on a growing number of web servers too. The reason is that it's 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 secure Operating Systems you can get, as security patches are released within days after a problem is discovered. This is possible since thousands of qualified developers are working for the Debian project on a voluntary basis and not only do they work on the OS itself, in addition, they provide 24/7 support via special e-mail lists and reply in 20 min to any question. In this light, you'll get an outstanding level of 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 - over 30,000. As the Operating System is free to use, you'll be able to modify both the core and any of the packages in any way you see fit. Also, you won't need to pay any kind of license fees.