Debian is a Linux distribution, that's widespread for desktop computers, yet it has been gaining popularity as a server OS over the last couple of years as well. It's very stable and safe, since fixes are released in a couple of days after an issue is discovered. Debian has a large support community, so if you have any kind of questions or issues with the OS or with some of the 30,000+ packages it comes with, you can contact the developers right away via email using special e-mail lists and receive a response in 15-20 min. The Debian community provides the top level of support possible, because all of your questions will be handled by the same people that built Debian in the first place, so they have an exhaustive knowledge of how their system works. Since the Operating System is 100% free to use and distribute, no license charges will be added to your web hosting fees and you'll be able to change the program code files and any other software which you install with no restrictions, to optimize the software environment for your sites. In addition, Debian performs really well on any hardware, because it is light and fast and doesn't require a lot of memory.