Debian is a Linux distribution, which is widely used for desktop machines, however it has been gaining in popularity as a server Operating System over the last couple of years as well. It's really stable and secure, as fixes are released within a few days after a problem is discovered. Debian has a huge support community, so when you have any questions or troubles with the OS or with any of the 30,000+ packages it features, you can contact the developers immediately through e-mail using special e-mail lists and receive a reply in 15-20 minutes. The Debian community provides the best level of support possible, since your questions will be handled by the very same people who made Debian in the first place, so they have an exhaustive knowledge of how their own system functions. As the Operating System is 100% free to use and distribute, no license costs will be added to your web hosting fees and you can change the code files and any other software which you install without any restrictions, so as to optimize the software environment for your sites. Furthermore, Debian performs very well on any hardware, as it is light and effective and doesn't require a lot of memory.