Nginx

Pronounced "Engine X"

"Developed by Igor Sysoev in 2002, Nginx was finally released to the public in 2004 and continues to grow in popularity every year. Nginx is an open-sourced, free HTTP server and reverse proxy. It can also act as an IMAP/POP3 server. Benefiting from being fully scalable, Nginx combines the use of a predictable small amount of memory with asynchronous architecture, resulting in a small memory footprint and low resource consumption. Nginx offers a high-performance, stable environment and is currently the chosen web server for WordPress, SourceForge, and TorrentReactor. Nginx is currently the 3rd most popular web server (behind Apache and IIS) with a market share of 7.65% (according to Netcraft’s March 2011 survey)."