Debian is a Linux distribution, that's widespread for desktop machines, however it has been gaining in popularity as a server Operating System during the last couple of years too. It is really stable and risk-free, since 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 problems with the OS or with any of the 30,000+ packages it is equipped with, you can easily contact the developers directly through email using special mailing lists and receive a reply within 15-20 min. The Debian community gives you the best level of support possible, because your questions will be handled by exactly the same people that created Debian in the first place, so they have an exhaustive knowledge of how their system functions. Since the Operating System is 100% free to use and distribute, no license costs will be included with your web hosting fees and you will be able to change the code files and any other software which you install without restrictions, to optimize the software environment for your websites. What's more, Debian performs very well on any kind of hardware, because it is light and efficient and doesn't need a lot of memory.