MediaWiki is the software used for Wikipedia and the other Wikimedia Foundation websites. Compared to other wikis, it has an excellent range of features and support for high-traffic websites using multiple servers (Wikimedia sites peak in the 60000+ requests per second range as of November 2008).
While quite usable on smaller sites, you may find you have to "roll your own" local documentation, and some aspects of configuration may seem overcomplicated because MediaWiki is primarily targeted as an in-house tool.
The MediaWiki software was written by:
The contributors hold the copyright to this work, and it is licensed under the terms of the GNU General Public License, version 2 or later (see http://www.fsf.org/licensing/licenses/gpl.html). Derivative works and later versions of the code must be free software licensed under the same or a compatible license. This includes "extensions" that use MediaWiki functions or variables; see http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins for details.
The Wikimedia Foundation currently has no legal rights to the software.
 Sections of code written exclusively by Lee Crocker or Erik Moeller are also released into the public domain, which does not impair the obligations of users under the GPL for use of the whole code or other sections thereof.
 MediaWiki makes use of the Sajax Toolkit by modernmethod, http://www.modernmethod.com/sajax/ which has the following license: 'This work is licensed under the Creative Commons Attribution License. To view a copy of this license, visit http://creativecommons.org/licenses/by/2.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.'
Many thanks to the Wikimedia regulars for testing and suggestions. The official website for MediaWiki is located at: http://www.mediawiki.org/
The code is currently maintained in a Subversion repository at svn.wikimedia.org. See http://www.mediawiki.org/wiki/Subversion for details.
Please report bugs and make feature requests in our Bugzilla system: https://bugzilla.wikimedia.org/
Documentation and discussion on new features may be found at:
Extensions are listed at: http://www.mediawiki.org/wiki/Category:Extensions
If you are setting up your own wiki based on this software, it is highly recommended that you subscribe to mediawiki-announce: https://lists.wikimedia.org/mailman/listinfo/mediawiki-announce
The mailing list is very low volume, and is intended primarily for announcements of new versions, bug fixes, and security issues.
A higher volume support mailing list can be found at: https://lists.wikimedia.org/mailman/listinfo/mediawiki-l
Developer discussion takes place at: https://lists.wikimedia.org/mailman/listinfo/wikitech-l
There is also a development and support channel #mediawiki on irc.freenode.net, and an unoffical support forum at www.mwusers.com.