Today, the folks at wikispaces.com have expanded their free wiki program to all educators, from K-12-PhD. The ad-free version of wikispaces' service used to cost $50 annually.

While at Columbia, we run two private label versions (http://wikispaces.columbia.edu for course work and http://wikischolars.columbia.edu for scholarly work), there have been instances where instructor's needs could not be accommodated by either of these platforms, necessitating the purchase of an a la carte wikispace. Now, these wikis can be obtained for free.

You can learn more about wikispaces' offer for higher ed on their website.