Let's look at some different wikis that are used in education.

Curriculum wiki - http://www.curriki.org/xwiki/bin/view/Main/WebHome
Media specialist - http://webtools4u2use.wikispaces.com/Webtools4U2Use
Web 2.0 resources - http://www.classroom20wiki.com/
Writing wikis - http://promiseintopractice.pbwiki.com/

Bud Hunt's Wiki: Created by Colorado teacher Bud Hunt and many educator-collaborators.
High School Online Collaborative Writing: Features the collaborative writing of various high school students.
Library Success: A Best Practices Wiki: Smart practices for libraries and the issues that impact them.
Westwood Schools Wiki: An online space for Westwood Schools' students that includes a variety of student projects, including an insightful analysis of Web 2.0 tools.
SwarmSketch: Just for fun: This site allows you to contribute a single line to a drawing and then vote for the inclusion of other lines in the drawing.
Warlick's Wiki: A wiki designed to support the presentations of education guru David Warlick, who speaks at Technology & Learning's Tech Forum events.
Wikibooks: Collection of textbooks that can be edited by anyone.