There are numerous wiki hosting sites. The following provide free hosting for education.

Wikispaces: If you are a teacher or student and you wish to create an advertisement free space that will be used for educational purposes then go to this link and join up for free. http://www.wikispaces.com/site/for/teachers If you would like to see more examples of classroom wikis created with wikispaces go here. If you want some online help or understanding then go to this link http://www.wikispaces.com/help+index

PBworks: This site is created in PBworks. PBworks, formerly pbwiki, is the largest wiki hosting site--primarily for the corporate world. 

Wetpaint Wetpaint wikis are "click-and-type" websites that are easy to start and even easier to expand, because you can invite anyone you like to add text, photos, links, and other content to your Wetpaint site. To add or edit content, all you need to do is click the EasyEdit button to activate an editing toolbar similar to what you'd see in programs like Microsoft Word. Then just start typing!

Educational Wikis

Second Life Educational Wiki--Sim Teach

Washington State University Wiki--Teaching Resources

Web 2.0 for Teaching and Learning Wiki