nginx (pronounced engine-x) is a free, open-source, high-performance HTTP server and reverse proxy, as well as an IMAP/POP3 proxy server. The nginx project started by Igor Sysoev in 2002 with a strong focus on high concurrency, high performance and low memory usage it was 2004 when the first public release become available. Nginx now holds more than 15% of the share across web server market.
Nginx is one of a handful of servers written to address the C10K problem which means ability to handle more than 10,000 simultaneous connections with a low memory footprint (~2.5 MB per 10k inactive HTTP keep-alive connections). Unlike traditional servers, Nginx doesn't rely on threads to handle requests. Instead it uses a much more scalable event-driven (asynchronous) architecture. This architecture uses small, but more importantly, predictable amounts of memory under load.
Even if one doesn't expect to handle thousands of simultaneous requests, he or her can still benefit from Nginx's high-performance and small memory footprint. Nginx scales in all directions: from the smallest VPS all the way up to clusters of servers.
Nginx powers several high-visibility sites, such as Netflix, Hulu, Pinterest, CloudFlare, Airbnb, WordPress.com, GitHub, SoundCloud, Zynga, Eventbrite, Zappos, Media Temple, Heroku, RightScale, Engine Yard and NetDNA.