Nginx is a popular light weight high performance web server and reverse proxy server.Igor Sysoev is the author of nginx.And it runs on all popular operating systems.Nginx can server as a load balancer it uses as asynchronous event-driven approach to handling requests instead of thread or process oriented approach.
It supports all the function apache web server does like reverse proxy,load balancing,URL rewriting,SSI,SSL Support,Fast CGI..etc.Nginx was found to be the third most widely used web server across all domains.You can find more information here.Most popular Rambler (Russian search engine )running in nginx.
It is a free open source software since the beginning with its sources licensed under 2-clause BSD-like license.