There are many examples of Wikis out there on the web.
The most well-known is Wikipedia http://www.wikipedia.org
Other examples of wikis
LibraryBlogsWikis http://www.seedwiki.com/wiki/libraryblogswikis/
A wiki that looks at how blogs and wikis are being used in Australian libraries
Charles Sturt Library Wiki http://csulibrary.wetpaint.com/page/Home
A wiki for the CSU Library staff and students
The Tolkien Wiki http://www.thetolkienwiki.org/wiki.cgi
A wiki for fans of the novels by JRR Tolkien
Al Upton and the Mini-Legends from Glenelg SA
http://alupton.edublogs.org/
Including a wiki for and by Primary students
The Open Guide to London http://london.openguides.org/
A travel guide to London, written by Londoners
Social Justice at Columbia University http://socialjustice.ccnmtl.columbia.edu/index.php/About
Collaborative website built as part of a course
Greek Tragedy Course Wiki - Skidmore College
http://academics.skidmore.edu/wikis/Greek_Tragedy/index.php/Main_Page
Wiki used as a group project presentation space
A Million Penguins http://www.amillionpenguins.com
A collaborative group novel wiki
Final Fantasy Wiki http://finalfantasy.wikia.com/wiki/Main_Page
A Final Fantasy database