Debian is a well-known Linux distribution, that is used mainly on desktop machines, however many people have started using it lately as a web server OS as well, because of the stability and security it supplies. The latter is a direct result the fact that Debian is developed and supported by a large number of qualified volunteers internationally, so any patch that is necessary to fix a certain issue is released in a matter of days. The same community can also help with any question or issue which a user may ever encounter, because they can be contacted via electronic mail using mailing lists and a competent response will be sent in no more than 15-20 min. The advantage here is that your queries will be responded to by the same people who have made the Operating System itself and they have an in-depth knowledge of how it works. As Debian is absolutely free, you won't have to pay any kind of license fees to use it and you can customize it as much as you want or need. This is valid for both the Operating System and the 30.000+ software packages that are available for it.