Igor Sysoev's HTTP server and reverse proxy. Runs ~33% of all active sites; still the dominant web server.
From Wikipedia
nginx is a web server that can also be used as a reverse proxy, load balancer, mail proxy and HTTP cache. The software was created by Russian developer Igor Sysoev and publicly released in 2004. nginx is free and open-source software, released under the terms of the 2-clause BSD license. A large fraction of web servers use nginx, often as a load balancer.