Linux/UNIX
According to netmarket.com, here is the latest operating system market share breakdown. If it weren't for Mac's 5% market share, Windows (91.09%) would pretty much be the only OS used.
For some reason Apple and Linux users have a tendency to believe that their computers are immune from viruses and malware. I am not sure who spread this rumor (some.
The Windows Security and Directory Services for UNIX Guide focuses on the use of Microsoft Windows Server 2003 or Windows 2000 Server Active Directory directory service to provide centralized authentication.