These wikis, listed below, were identified by http://www.wikimatrix.org,  as being ideal for educators. They all allow for WYSIWYG editing, are hosted by someone else (so you don’t have to manage the software), allow you to track the history of the wiki and revert to past changes, and are FREE!

@Wiki, BrainKeeper, CentralDesktop, ClearWiki, Confluence, Cospire, DekiWiki, EditMe, Groupswiki, Intodit, Luminotes, Metadot Wiki, Netcipia, nexdo, PBwiki, SamePage, ScribbleWiki, SeedWiki, Socialtext, Springnote, Wetpaint, Wikispaces and Zoho Wiki

I've used Moodle, Peanut Butter Wiki (pbwiki) and WetPaint successfully .with students in grades 7-20. (If you go to that Moodle website you can log in as a guest and have access to a free course in how to use Moodle!)

If you use WetPaint you have to create the wiki then send them an email that it is an educational wiki. They will remove the ads, which can be inappropriate for a K-12 environment, after receiving your email.

Wetpaint has an easier way to make editing comments, and is somewhat simpler/more static than peanutbutter wiki. This can help organizationally.

Moodle provides an entire free online course software system, which includes wikis as one of its many features.

Peanut Butter wiki is the most intuitive of the three in terms of its editing structure, but there is the potential for student to get lost in the material if many are creating new pages simultaneously.