Windows Server Essentials!

ClearOS is a family of operating systems for the enterprise, small business, and home. There are three editions to choose from depending on your needs and suitable to your abilities. They are: ClearOS Enterprise, ClearOS Home, and ClearOS Core.

Zentyal is a native Microsoft Active Directory® implementation on Linux incorporating all the network services required in a small and medium business environment: Directory & Domain Server, Mail Server, Gateway and Infrastructure Server.

Univention Corporate Server is a pre-configured Linux server operating system free of charge. It's the best fit to provide Microsoft Server-like services on the cloud or on the premises, to run and operate corporate IT environments with Windows- and Linux-based clients and to extend those environments with proven enterprise software, also either on the cloud or on the premises.

Windows Server 2012, codenamed "Windows Server 8", is the sixth release of Windows Server. It is the server version of Windows 8 and succeeds Windows Server 2008 R2. Two pre-release versions, a developer preview and a beta version, were released during development. The software was generally available to customers starting on September 4, 2012.

The Amahi Home Server is designed for home and small office environments where the technical support staff do not want the job!

Untangle is a platform for deploying network based applications. The platform unites these applications around a common GUI, database and reporting. Applications on the Untangle platform inspect network traffic simultaneously, which greatly reduces the resource requirements of each individual application. Untangle can act as a complete unified threat management (UTM) box, going beyond just a basic firewall or router. It's aimed at small-to-medium businesses and enthusiast home users. It runs in line as a network gateway, so it allows control of all devices connecting to the network. Untangle offers a free, open source Lite Package as well as a full-featured, commercial Premium Package. Untangle has about 40,000 installations worldwide protecting over 2 million people in businesses, schools and other organizations. »

MDaemon Messaging Server software for Windows can be deployed in virtual, hosted cloud or private on-premise environments. It is used by businesses in over 90 countries and over 25 languages worldwide.

The Linux operating system for educational establishments.

SME Server is a leading distribution for small and medium enterprises. It stands apart from the competition by shipping with most common functionality preconfigured and features a number of popular additional enhancements in the form of downloadable contributions. It is published under the GPL license.

Just like that, your Mac is a powerful server.