Nginx

From http://wiki.nginx.org/

Nginx ("engine x") is a free, open-source, high-performance HTTP server and reverse proxy, as well as an IMAP/POP3 proxy server. Igor Sysoev started development of Nginx in 2002, with the first public release in 2004. Nginx now hosts nearly 7.65% (22.8M) of all domains worldwide.

Nginx is known for its high performance, stability, rich feature set, simple configuration, and low resource consumption.

Nginx is one of a handful of servers written to address the C10K problem. 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 you don't expect to handle thousands of simultaneous requests, you 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 WordPress, Hulu, Github, Ohloh, SourceForge and TorrentReactor.

TestLink uses Nginx as http server.

A http server supports static files, such as js, html, css, images etc.

A app server supports dynamic files, such as php, ruby, c#, java etc.