Many small and large organizations are getting CMS functionality from wikis.

Indeed there may be more of these CMS-lite tools than there are CMS themselves. Many Wiki sites have surpassed 10,000 downloads and thousands of registered users.

See the Information's Architect's Wiki for an example of how wikis are used, with a lot of valuable information for designing a CMS. An even better wiki is Simon Willison's companion wiki to the CSS-Discuss mailing list.

Also see the CMS Wiki where you can contribute to a knowledge base for Content Management Systems.

Major Wikis ...
Wiki	Ward Cunnigham original Wiki
Wikipedia	Wikipedia
MediaWiki	The Wikipedia Software
JSPWiki	A JSP Wiki
MoinMoin Wiki	A Python Wiki
Open Wiki	www.openwiki.com
PhpWiki	A PHP Wiki
PikiPiki	Another Python Wiki
Snip Snap	www.snipsnap.org
Swiki	A Squeak Wiki
Tiki	www.tikiwiki.org
Twiki	www.twiki.org
Zwiki	A Zope Wiki