Linux Server
A Linux server is very flexible.
In the business environment the flexibility of Linux allows it to take on the roles of: Inranet Server, File Server, Document Server and Backup Server just to name a few.
The reliability of Linux is proven beyond doubt.
The business that uses the advantage of the Open Source Licence that Linux is released under will appreciate no more licence costs and restrictions, as your business grows. There is no need to buy more connections to the server (called cals by MicroSoft) as many of the business needs are legal and available. The business does not need staff to monitor licence compliance, a hidden on going cost to most business.