Debian is a Linux distribution, that's widely used for desktop computers, but it has been gaining popularity as a server OS during the last few years too. It is really stable and protected, as fixes are released in just a few days after an issue is discovered. Debian has a large support community, so when you have any kind of questions or troubles with the OS or with some of the 30,000+ packages it is equipped with, you'll be able to contact the developers right away via electronic mail using special mailing lists and receive a response in just 15-20 min. The Debian community provides the top level of support possible, since all your questions will be handled by the same people that made Debian in the first place, so they have a detailed knowledge of how their own system works. Since the Operating System is free to use and distribute, no license charges will be added to your hosting fees and you'll be able to customize the program code files and any additional software which you install without any restrictions, so as to optimize the software environment for your sites. What's more, Debian performs really well on any kind of hardware, because it is light and efficient and doesn't need a lot of memory.