WikiTools

Wikispaces – offers free spaces for teachers here:
http://www.wikispaces.com/site/for/teachers100K

Wikia
http://www.wikia.com/wiki/Wikia

PBWiki
http://pbwiki.com/

Wet Paint
http://www.wetpaint.com/

In fact, there are so many Wiki tools available that there’s a Wiki that allows you to compare different tools here:
http://www.wikimatrix.org/