Debian is an OS which uses the Linux kernel. Though it is usually set up on desktop machines, it is being used on a growing number of web servers as well. The reason is that it is really stable, so a server running it won't need to be restarted for months. Debian is also regarded as being one of the most secure Operating Systems to be found, as security patches are released within days after an issue is discovered. This is possible since thousands of well-experienced 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 respond within 20 min to any question. Thus, you will obtain top-notch support directly from the developers who know their own platform in detail, and not from a third-party support team. Debian is also among the Linux distributions with the highest number of packages - over 30,000. Since the Operating System is completely free to use, you'll be able to modify both the core as well as any of the packages in the way you see fit. Furthermore, you will not have to pay any kind of license fees.