Debian is a Linux distribution, that is widely used for desktop machines, but it has been gaining popularity as a server Operating System during the last couple of years as well. It's really dependable and secure, because fixes are released in just a few days after an issue is discovered. Debian has a vast support community, so when you have any kind of questions or problems with the OS or with any of the 30,000+ packages it features, you're able to contact the developers immediately through electronic mail using special e-mail lists and receive a reply within 15-20 min. The Debian community provides the top level of support possible, because all your questions will be handled by the same people who created Debian in the first place, so they have an exhaustive knowledge of how their own system works. Because the Operating System is absolutely free to use and distribute, no license charges will be included with your hosting fees and you'll be able to modify the code files and any other software which you install with no restrictions, to optimize the software environment for your sites. What's more, Debian performs really well on any kind of hardware, because it's light and fast and doesn't require a lot of memory.
