Apache (Web Server) is open source HTTP server software,
& is also number 1 HTTP server on the Internet.
About Apache from apache.org:
"The Apache HTTP Server Project is a collaborative software development
effort aimed at creating a robust, commercial-grade, featureful,, and
freely-available source code implementation of an HTTP (Web) server. The
project is jointly managed by a group of volunteers located around the
world, using the Internet and the Web to communicate, plan, and develop
the server and its related documentation. This project is part of the
Apache Software Foundation. In addition, hundreds of users have
contributed ideas, code, and documentation to the project. This file is
intended to briefly describe the history of the Apache HTTP Server and
recognize the many contributors.