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
Caddy - The HTTP/2 Web Server with Automatic HTTPS
Cherokee - High-performance web server
WEBrick - WEBrick is a Ruby library providing simple HTTP web server services.