Linux
Free, Open, Stable, Robust, Ubiquitous
Linux is an operating system as are Apple's OSX, Microsoft's Vista, XP and 2K3 Server.
Linux will run servers, and desktops and the chances are that it is already running some aspect of your business.
We have 12 years experience of providing rock solid Linux systems which keep churning out results for a fraction of the cost of other operating systems.