Google Web Server
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According to Netcraft's Web Server Survey in October 2009, Google Web Server makes Google the #4 developer of web server software among all domains, with 6.00% market share, and the #3 developer among active sites, with 13.56% market share. 
Google is intentionally vague about GWS, saying only that it is a custom-built server that runs on Linux. There is some speculation that GWS is a modified version of Apache, which would be plausible given Google's proclivity for modifiying open source software for its own internal use. The web server used can be checked by typing GET command as shown below on command prompt.
GET http://www.google.com HTTP/1.1