Two of the main wiki-making sites are Pbwiki and Wetpaint. Both allow you to set up individual logins, make pages private, decide who’s allowed to post, and who can edit who’s work. Also worth a look are Versionate, Google Sites, Wikispaces, and Wikia. All are potentially very useful for setting up a class assignment, collaborating on a research project, or crowdsourcing a set of instructions.