Debian is a well-known Linux distribution, that is used primarily on PC's, but a lot of people have started using it lately as a web server Operating System as well, because of the stability and protection it offers. The latter is a consequence of the fact that Debian is developed and supported by thousands of qualified volunteers all around the world, therefore any patch that is necessary to fix a certain issue is released within a few days. The very same community will also help you with any question or trouble that a user may ever experience, since they can be contacted via email using mailing lists and a competent response will be sent in around 15-20 min. The advantage here is that all your questions will be answered by the same people who have made the Operating System itself and they have a detailed knowledge of how it works. As Debian is free, you will not need to pay any license fees to use it and you'll be able to customize it as much as you want or need. This is valid for both the Operating System and the 30.000+ software packages which are available for it.