Wikipedia defines a Content Management System (CMS) as follows:
A content management system is a computer software system for organizing and facilitating collaborative creation of documents and other content. A content management system is sometimes a web application used for managing websites and web content, though in many cases, content management [...]

Vote to indicate your choice as the best Open Source Content Management System.

The following information was compiled from the Wordpress site.
WordPress started in 2003 with a single bit of code to enhance the typography of everyday writing and with fewer users than you can count on your fingers and toes. Since then it has grown to be the largest self-hosted blogging tool in the [...]

About Drupal
This is a useful compilation of information taken from Drupal’s site at http://www.drupal.org. Drupal is software that allows an individual or a community of users to easily publish, manage and organize a great variety of content on a website. Tens of thousands of people and organizations have used Drupal to [...]

About e107
A compilation of information from e107’s Site at http://www.e107.org
e107 is a content management system written in php and using the popular open source mySQL database system for content storage. It’s completely free and totally customisable, and in constant development.
Quick FAQ
Q. So why is it called e107?
A. It just is. Stupid name, maybe, [...]

About Joomla
A summary of information gleaned from http://www.joomla.org. Joomla turned 1 year old on 1 Sep 2006.
The story behind Joomla! is long and rich in detail. There are many different perspectives. The story below is the author’s on Joomla. Since it describes what Joomla is about as no other I thought I would post [...]

About Mambo
More information can be found at http://www.mamboserver.com. You can read the Joomla! article for more information. Both sites are diplomatic about the rift but there is some definite tension here. Choice of any 5 of the top CMS’s boils down to a “religious” decision in the end.
What is [...]

About Plone
Learn more about Plone by visiting http://www.plone.org which is where the information below is collated from. Why is it all of these Content Management Systems have weird names?
What is Plone?
Plone is a ready-to-run content management system that is built on the powerful and free Zope application server. Plone is easy to [...]

About Xoops
Visit http://www.xoops.com for more information. I collated information from that site.
XOOPS is a program that allows administrators to easily create dynamic websites with great content and many outstanding features. It is an ideal tool for developing small to large dynamic community websites, intra company portals, corporate portals, weblogs and [...]