"PBwiki knows that budgets are going to be tight, and we don’t want a lack of funds to deter educators from using collaborative technologies like wikis with their students.
To this end, PBwiki is giving each teacher or librarian who signs up as part of the program between August 18 and November 31, a free premium wiki (regular price: $250/year).
For example, if you sign up for a free wiki on November 1, you would receive a premium upgrade that lasts until October 31, 2009. Each fall, PBwiki plans to offer additional programs that will let participants retain their premium status free of charge."
You can learn more and sign up for the program by clicking here.