“Security, speed, compliance and flexibility” this is the description of Lighttpd(Light Http) on the official website. It is an open-source web server optimized for speed conections. It was originally written by Jan Kneschke as a proof-of-concept of the C10k problem* and now it gained worldwide popularity.
The low memory footprint, small CPU load and speed optimizations make lighttpd suitable for low servers, meaning servers the suffer load problems.
It is distributed under the BSD license and it runs on Linux and any other Unix-like operating systems (like Microsoft Windows) where it can be controlled with the Lighty Tray program.
Lighttpd supports the FastCGI, SCGI and CGI interfaces to external programs, leting every web applications written in any programming language to be used with this server.
Web 2.0 sites like YouTube, Wikipedia or Meebo use this open-source web server.
*C10k problem is the numeronym given to a limitation that most web servers curently have a limitation of 10 000 simultaneous connections. Althogh there are some web servers that can handle more then 10 thousand client connections.