I'm considering doing a wiki-based assignment for a course (as a form of class participation, participation in the wiki contributing a small but as-yet-undetermined percent of the final grade). Does anyone have any recommendations for a free wiki hosting site that's very user-friendly -- something even students with little wiki experience could easily jump into (and have a bit of fun with)? Do any of the following sites have advantages and disadvantages that you know of?
Anything anyone might know from personal experience or even hearsay would be welcome. The big desiderata here are (1) ease of use; (2) good help features (tutorials, or whatever) that students could use; and (3) any features that might make it especially fun or enjoyable to work with.
P.S. I suppose I should be more specific about what I have in mind; I'm thinking about having students build, collaboratively, a sort of reference site out of class notes, their own work, etc. But this is all just at the bare-idea stage, so if you've come across excellent educational uses of wikis other than this, I'd like to know that, too.