The most popular web server has always been OSS/FS since such data have been collected. For example, Apache is the current #1 web server. Netcraft’s statistics on web servers have consistently shown Apache (an OSS/FS web server) dominating the public Internet web server market ever since Apache grew into the #1 web server in April 1996. Before that time, the NCSA web server (Apache’s ancestor) dominated the web from August 1995 through March 1996 - and it is also OSS/FS.

GNU/Linux is the #2 web serving OS on the public Internet (counting by physical machine), according to a study by Netcraft surveying March and June 2001. Some of Netcraft’s surveys have also included data on OSes; two 2001 surveys (their June 2001 and September 2001 surveys) found that GNU/Linux is the #2 OS for web servers when counting physical machines (and has been consistently gaining market share since February 1999). As Netcraft themselves point out, the usual Netcraft web server survey (discussed above) counts web server hostnames rather than physical computers, and so it doesn’t measure such things as the installed hardware base. Companies can run several thousand web sites on one computer, and most of the world’s web sites are located at hosting and co-location companies.

Therefore, Netcraft developed a technique that indicates the number of actual computers being used as Web servers, together with the OS and web server software used (by arranging many IP addresses to reply to Netcraft simultaneously and then analyzing the responses). This is a statistical approach, so many visits to the site are used over a month to build up sufficient certainty. In some cases, the OS detected is that of a “front” device rather than the web server actually performing the task. Still, Netcraft believes that the error margins world-wide are well within the order of plus or minus 10%, and this is in any case the best available data.

Before presenting the data, it’s important to explain Netcraft’s system for dating the data. Netcraft dates their information based on the web server surveys (not the publication date), and they only report OS summaries from an earlier month. Thus, the survey dated “June 2001” was published in July and covers OS survey results of March 2001, while the survey dated “September 2001” was published in October and covers the operating system survey results of June 2001.

Here’s a summary of Netcraft’s study results:

Much depends on what you want to measure. Several of the BSDs (FreeBSD, NetBSD, and OpenBSD) are OSS/FS as well; so at least a part of the 6.1% for BSD should be added to GNU/Linux’s 29.6% to determine the percentage of OSS/FS OSes being used as web servers. Thus, it’s likely that approximately one-third of web serving computers use OSS/FS OSes. There are also regional differences, for example, GNU/Linux leads Windows in Germany, Hungary, the Czech Republic, and Poland.

Well-known web sites using OSS/FS include Google (GNU/Linux) and Yahoo (FreeBSD).

source : http://www.dwheeler.com/oss_fs_why.html