By Web Server Software, I mean the software used to serve websites/pages. This includes databases, operating systems and other software that is involved in that process. On the proprietary side, there are two options. Proprietary Unix, and Microsoft Windows, and associated Microsoft Software. The current version of MS Server in use is Server 2008. Microsoft has web server software called IIS, and it's database server product is MS SQL server, which people use for far more than just serving web site data. The primary web development framework used in this environment is .NET. Proprietary UNIX has dwindled greatly in popularity with the increasing popularity of Linux. On top of proprietary UNIX, people will generally run associated open source server software for web, database and development frameworks. On the open source side, Linux is by far the most popular, with BSD in second place. Both Linux and BSD come in several flavors (or distributions.) Apache is by far the most popular web server software. MySQL and PostgreSQL are the open source database systems most in use for web servers, with PostgreSQL being a pretty distant second to MySQL. Other database systems (such as NoSQL variants) are increasing in popularity, but are pretty far down from MySQL as well. Also, it is possible to run Apache, most varieties of open source databases and web frameworks on Windows, and that is not uncommon. It's hard to know what the market share of server operating systems are, because there are different ways to measure it. You can measure how many units are sold. By that measure, Windows is first at about 49-67%, Linux is second at 16-23%, and proprietary UNIX is third at 7-22%. That underestimates things like self-installed OS systems (standard with Linux), as well as VPS systems. If you measure by surveying publicly accessible websites, you get Linux first at 41%-74%, Windows second at 20-42% and proprietary UNIX third at 2-5%. This underestimates servers inside enterprises. (source: wikipedia) From my perspective, the underestimation of self-installed and VPS systems by the first measure far outweighs the underestimation of enterprise servers, because plenty of organizations and enterprises also install Linux behind the firewall. It would make sense to me that the true number is much closer to the estimation by publicly accessible websites, rather than the unit sales estimation. So on the OS side, Linux does look like it wins. Apache is far and away the most popular web server software. It is way ahead of IIS. The most recent data from Netcraft shows that Apache has 63% of web servers, compared to 19% for IIS. Also, Apache is showing a clear upward trend, and IIS a clear downward trend.