Amongst the the most popular network operating systems are:

• Windows Server 2003 — A popular network operating system because it addresses most of a network administrator’s needs very well. Microsoft is, of course, a well-established vendor, and many devices and programs are compatible with its systems. Its large market share guarantees that technical support—whether through Microsoft, private developer groups, or third-party newsgroups—is readily available.
• UNIX-type of systems — Interoperate well with other NOS, when installing the Samba application they allow the exchange information with Windows servers by using Windows file system and file access protocols.
• Linux — An open-source NOS, popular in academic circles, mirrors many features of UNIX-type NOSs.
• Mac OS X Server — A proprietary NOS from Apple Computer that is based on a version of UNIX.
• NetWare — Novell released its first NetWare NOS in 1983, and it quickly became the NOS of choice for many businesses. In subsequent years, Novell refined NetWare to make it run over TCP/IP (in addition to IPX/SPX) and to provide a graphical user interface for its file and resource management, plus a broad range of Internet services, fault-tolerance techniques, and seamless integration with other operating systems.  In the last decade, NetWare has lost approximately 80% of its previous market share to more popular NOSs, such as Windows Server 2003, UNIX, and Linux