To date, Wikispaces has given away more than 980,000 free advertising-free wikis to K-12 classrooms. Wikispaces has the stated goal of giving away two million advertising-free wikis to educators. To that end, Wikispaces is now giving away advertising-free wikis to higher education users. These wikis never expire and are full of the same features you would find if you were purchasing one of Wikispaces’s premium $50/year wikis for a business. The free wikis for educators can be made private or public.
If you would like to learn how to create a free wiki on Wikispaces, please see my Wikispaces tutorial.