Apache is a freely available, and highly popular, open-source web server.
Originally, Apache was designed for Unix. Now versions are available for most operating systems including Windows, OSX and Linux. There are also numerous add-ons and tailored versions of the server using the Apache module API. The name Apache comes from its origins as a series of “patch files.”
Read Webmonkey’s Apache for Beginners article for more details about Apache.
Information and downloads can be found at the Apache Software Foundation website.