Debian is a Linux distribution, that's preffered for desktop machines, but it's been gaining popularity as a server Operating System during the last couple of years too. It's really stable and secure, 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 features, you are able to contact the developers directly through email using special e-mail lists and receive a reply within 15-20 minutes. The Debian community provides the top level of support possible, as your questions will be handled by the same people who made Debian in the first place, so they have a thorough knowledge of how their own system functions. As the Operating System is free to use and distribute, no license costs will be included with your hosting fees and you will be able to modify the program code files and any other software that you install with no restrictions, so as to optimize the software environment for your sites. In addition, Debian performs really well on any hardware, as it is light and quick and doesn't need a lot of memory.