HTTP web server
Been used in over 100 million websites and has been a huge part of the WWW growth
Apache is a multi-platform server that primarily supports Unix-like operating systems like UNIX, FreeBSD, Linux, and Solaris. It is also compatible with Mac OS X and Microsoft Windows. Apache was developed by Robert McCool in 1995, and it was first released in 1995. Its most recent stable release, 2.2.19, was released on May 22, 2011. Apache is free software written in the C programming language and distributed under the Apache license 2.0.