nginx - A high performance free open source web server powering busiest sites on the Internet.
lighttpd - A secure, fast, compliant, and very flexible web-server that has been optimized for high-performance environments
Apache HTTP Server - Apache httpd has been the most popular web server on the Internet since April 1996
Cherokee - High-performance web server
Microsoft IIS - Internet Information Services is a web server for Microsoft Windows
mongrel - A small fast HTTP library and server that runs Rails, Camping, Nitro and Iowa apps.