Apache

A open source HTTP server for Unix, Windows NT, and other platforms. Apache was developed in early 1995, based on code and ideas found in the most popular HTTP server of the time, NCSA httpd 1.3. It has since evolved to rival (and probably surpass) almost any other Unix based HTTP server in terms of functionality, and speed. Since April 1996 Apache has been the most popular HTTP server on the Internet, in May 1999 it was running on 57% of all web servers.

It features highly configurable error messages, DBM-based authentication databases, and content negotiation.