Most commercial servers use UNIX, Linux, Solaris, or Windows Server operating systems. A few have used Macintosh Server. Apple recently stopped selling its high-end Xserve hardware but continues to support the server software that runs on the Mac mini server and other Apple hardware.
In recent years, in-home servers have also become popular, with most of them running either Linux or Windows Home Server operating systems. In-home servers are popular for storing and sharing media files, backups, shared calendars, file sharing, and more. I have seen a number of in-home servers used with large-screen high-def television screens. Serving video and MP3 files to television sets and stereos throughout the home is a great application, as is providing storage space for backup files of all of a family's computers.
Writing in the MacWorld web site, Kirk McElhearn points out that Apple is about to move into the home server business as well. The soon-to-be-released Lion operating system will add server functionality that can be run on a new or even some older Macs. In addition, unlike Windows Home Server, the computer does not need to be dedicated solely for operation as a server. You should be able to use any modern Macintosh simultaneously as a normal desktop computer and as a home server. With Lion, you won’t need to buy a Mac mini server or buy Mac OS X Server software on its own. Lion Server will be able to handle all of the tasks that you need to run a simple server for media files, backups, shared calendars, file sharing, and more. If Apple can simplify the configuration a bit, this could bring these server features to those who want to invest in an extra Mac, or even use an older Mac (as long as it can run Lion) as a digital nerve center in their home.
Actually, almost all that functionality is already built into the current Snow Leopard release of OS X although it but it involves a fair amount of tweaking to enable everything. The new Lion release promises to simplify the installation and configuration.
You can read Kirk McElhearn's article at http://www.macworld.com/article/158157/2011/02/lion_osxserver.html#lsrc.nl_mwweek_h_cbstories