The Apache Software Foundation is a non-profit group that manages many open source software development efforts.  Most notable is the Apache Web Server.  The Apache Web Server is by far the most deployed HTTP server out there.  Apache is a very robust web server with countless modules, plugins, and extensions to make it do just about anything you with with a web server.  I have even heard of an extension that allows you to run ASP.NET sites through Apache.

The Apache Software Foundation also manages other web technology projects.  One of the more popular Apache projects is Jakarta Tomcat.  Tomcat is the most widely used servelet container for serving JSP pages.