nginx - A high performance free open source web server powering busiest sites on the Internet.
Apache HTTP Server - Apache httpd has been the most popular web server on the Internet since April 1996
lighttpd - A secure, fast, compliant, and very flexible web-server that has been optimized for high-performance environments
Wt - Wt (pronounced as witty) is a C++ library for developing web applications.
Cherokee - High-performance web server
mongrel - A small fast HTTP library and server that runs Rails, Camping, Nitro and Iowa apps.