The Apache HTTP Server Project is an effort to develop and maintain an
open-source HTTP server for various modern desktop and server operating
systems, such as UNIX and Windows NT. The goal of this project is to
provide a secure, efficient and extensible server which provides HTTP
services in sync with the current HTTP standards.
The 2.x branch of Apache Web Server includes several improvements like
threading, use of APR, native IPv6 and SSL support, and many more.

WWW: http://httpd.apache.org/