Debian is an OS that uses the Linux kernel. Though it is usually installed on desktops, it is being used on a growing number of web servers as well. The reason is that it's 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 safe Operating Systems you can get, as security patches are released within days after a problem is found. This is possible since thousands of well-trained developers are working for the Debian project on a voluntary basis and not only do they work on the OS itself, they also provide 24/7 support via special mailing lists and reply within 20 min to any question. Thus, 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 - more than 30,000. Since the Operating System costs nothing to use, you will be able to modify both the core as well as any of the packages in the way you see fit. In addition, you won't need to pay any license fees.