Debian is an OS that uses the Linux kernel. Though it is usually installed on personal computers, it is being used on a growing number of web servers too. The reason is that it is very stable, so a server running it will not have to be restarted for months. Debian is also regarded as being among the most secure Operating Systems to be found, due to the fact that security patches are released within days after a problem is discovered. This is possible since thousands of qualified 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 through special e-mail lists and reply in 20 min to any kind of question. Thus, you will get 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 releases 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 the way you see fit. Also, you will not have to pay any license fees.