Apache 2, the successor to Apache 1.

Apache HTTP Server is the most used Web server software worldwide.

The Apache HTTP Server is an open-source HTTP server for modern operating
systems including UNIX, Microsoft Windows, Mac OS/X and Netware. The goal of
this project is to provide a secure, efficient and extensible server that
provides HTTP services observing the current HTTP standards. Apache has been
the most popular web server on the Internet since April of 1996.

This version of httpd (2.4) is a major release of the stable branch, and
represents the best available version of Apache HTTP Server. New features
include Smart Filtering, Improved Caching, AJP Proxy, Proxy Load Balancing,
Graceful Shutdown support, Large File Support, the Event MPM, and refactored
Authentication/Authorization.

The prominent new feature in Apache 2 were:
- hybrid multiprocess, multithreaded mode for improved scalability
- multiprotocol support
- stream filtering
- IPv6 support
- new module API

See /usr/share/doc/packages/apache2/, http://httpd.apache.org/, and
http://httpd.apache.org/docs-2.4/upgrading.html.