Apache - Web Server
Apache has been the most popular web server on the Internet since April of 1996. The December 2000 Netcraft Web Server Survey found that over 60% of the web sites on the Internet are using Apache (over 62% if Apache derivatives are included), thus making it more widely used than all other web servers combined.
The Apache 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.
This information was gathered from http://httpd.apache.org/