The Apache HTTP Server is a widly used open source web server platform for many current operating systems such as UNIX, Windows, Linux, Mac OS X, Solaris and more. The Apache Web server, first released to the public in April of 1995, is a project of The Apache Software Foundation. Apache was partly responsible for the early expansion and development of the World Wide Web. The source code for Apache is freely-available to anyone that requires it for modification, implementation of additional features and services, or to be used in the creation of new applications.
Many modules for Apache are available which add extra features and functionality including CGI, SSI, URL redirection, etc.
The Apache HTTP Server can be download free of charge at http://httpd.apache.org/download.cgi.