Wikipedia: The biggest multilingual free-content encyclopedia on the Internet. Over two million articles and still growing.
LittleWiki: LittleWiki is a Web site where you can create public and private wiki pages. A Wiki is a Web site where anyone can enter and edit content. That means you can post and edit information about virtually anything you wish, and you can read what others have posted too.
Wetpaint: Wetpaint powers websites that tap the power of collaborative thinking. The heart of the Wetpaint advantage is its ability to allow anyone - especially those without technical skill - to create and contribute to websites written for and by those who share a passion or interest. To do this, Wetpaint combines the best aspects of wikis, blogs, forums and social networks so anyone can click and type on the web.
JotSpot: Create and share wiki pages with a WYSIWYG editor via Jotspot's secure wiki hosting. Install various "applications" to refine your wiki content - project manager, company directory, blog, forum, poll, calendar, and more - then invite others to contribute
PBWiki: Make a free, password protected wiki as easily as a peanut butter sandwich. Easily upload HTML, link files to your pages, integrate RSS, and more.