Unix-like operating systems
A customized KDE desktop running under Linux.
A customized KDE desktop running under Linux.

The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "UNIX" is a trademark of The Open Group which licenses it for use with any operating system that has been shown to conform to their definitions. "Unix-like" is commonly used to refer to the large set of operating systems which resemble the original Unix.

Unix systems run on a wide variety of machine architectures. They are used heavily as server systems in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as Linux and BSD, are popular in these areas. The market share for Linux is divided between many different distributions. Enterprise class distributions by Red Hat or SuSe are used by corporations, but some home users may use those products. Historically home users typically installed a distribution themselves, but in 2007 Dell began to offer the Ubuntu Linux distribution on home PCs. Linux on the desktop is also popular in the developer and hobbyist operating system development communities. (see below)

Market share statistics for freely available operating systems are usually inaccurate since most free operating systems are not purchased, making usage under-represented. On the other hand, market share statistics based on total downloads of free operating systems are often inflated, as there is no economic disincentive to acquire multiple operating systems so users can download multiple, test them, and decide which they like best,

Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's hardware. Others, such as Solaris, can run on multiple types of hardware, including x86 servers and PCs. Apple's Mac OS X, a hybrid kernel-based BSD variant derived from NeXTSTEP, Mach, and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS.

Open source

See POSIX -- full Unix interoperability heavily depends on full POSIX standards compliance. The POSIX standard applies to any operating system.

Over the past several years, the trend in the Unix and Unix-like space has been to open source operating systems. Many areas previously dominated by UNIX have seen significant inroads by Linux; Solaris source code is now the basis of the OpenSolaris project.

The team at Bell Labs that designed and developed Unix went on to develop Plan 9 and Inferno, which were designed for modern distributed environments. They had graphics built-in, unlike Unix counterparts that added it to the design later. Plan 9 did not become popular because, unlike many Unix distributions, it was not originally free. It has since been released under Free Software and Open Source Lucent Public License, and has an expanding community of developers. Inferno was sold to Vita Nuova and has been released under a GPL/MIT license.

Mac OS X

Mac OS X is a line of proprietary, graphical operating systems developed, marketed, and sold by Apple Inc., the latest of which is pre-loaded on all currently shipping Macintosh computers. Mac OS X is the successor to the original Mac OS, which had been Apple's primary operating system since 1984. Unlike its predecessor, Mac OS X is a UNIX operating system built on technology that had been developed at NeXT through the second half of the 1980s and up until Apple purchased the company in early 1997.

The operating system was first released in 1999 as Mac OS X Server 1.0, with a desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, five more distinct "end-user" and "server" editions of Mac OS X have been released, the most recent being Mac OS X v10.5, which was first made available in October 2007. Releases of Mac OS X are named after big cats; Mac OS X v10.5 is usually referred to by Apple and users as "Leopard".

The server edition, Mac OS X Server, is architecturally identical to its desktop counterpart but usually runs on Apple's line of Macintosh server hardware. Mac OS X Server includes workgroup management and administration software tools that provide simplified access to key network services, including a mail transfer agent, a Samba server, an LDAP server, a domain name server, and others.

Microsoft Windows

The Microsoft Windows family of operating systems originated as a graphical layer on top of the older MS-DOS environment for the IBM PC. Modern versions are based on the newer Windows NT core that was originaly intended for OS/2 and borrowed from VMS. Windows runs on 32-bit and 64-bit Intel and AMD processors, although earlier versions also ran on the DEC Alpha, MIPS, Fairchild (later Intergraph) Clipper and PowerPC architectures (some work was done to port it to the SPARC architecture).

As of September 2007, Microsoft Windows holds a large amount on the worldwide desktop market share. Windows is also used on servers, supporting applications such as web servers and database servers. In recent years, Microsoft has spent significant marketing and research & development money to demonstrate that Windows is capable of running any enterprise application, which has resulted in consistent price/performance records (see the TPC) and significant acceptance in the enterprise market.

The most widely used version of the Microsoft Windows family is Windows XP, released on October 25, 2001.

In November 2006, after more than five years of development work, Microsoft released Windows Vista, a major new version of Microsoft Windows which contains a large number of new features and architectural changes. Chief amongst these are a new user interface and visual style called Windows Aero, a number of new security features such as User Account Control, and new multimedia applications such as Windows DVD Maker.