2. Apache

Apache is the leading internet web server, with over 60% market share, according to the Netcraft survey. Several key factors have contributed to Apache's success:

• The Apache license. It is an open source, BSD-like license that allows for both commercial and non-commercial usage of Apache.
• Talented community of developers with a variety of backgrounds and an open development process based on technical merits.
• Modular architecture.
• Portable: Apache runs on nearly all flavors of Unix (and Linux), Windows, BeOs, mainframes...

The following websites use Apache or derivatives. Chances are that if Apache is good enough for them, it is also good enough for you :)

• Amazon.com
• Yahoo!
• W3 Consortium
• Financial Times
• Network solutions
• MP3.com
• Stanford

>From the Apache website:

The Apache Project is a collaborative software development effort aimed at creating a robust, commercial-grade, featureful, and freely-available source code implementation of an HTTP (Web) server.

The Apache project has grown beyond building just a web server into other critical server side technologies like Java or XML. The Apache Software Foundation, described in the next section serves as an umbrella for these projects.