One of the problems with many "free" Internet services is that they are supported by advertising. Generally that is a pretty minor issue and for most applications where "free" as in no out of pocket costs is a requirement its a non-issue. In schools though this can be a huge issue. In fact it can be a showstopper. You just cannot have students using a site where you have no control over content.
So I was pleased to find out the other day that the WetPaint wiki site is now allowing educational wikis to be advertising free. I don't know much more about them than I found out by looking around for a while but they do seem to have a thriving educational contingent over there. They recently named a teacher as an Educational Ambassador which seems like a second step in a positive direction.
You can get more information at their Wiki in Education portal. There are limitations and a qualification process but it sounds reasonable.
The email I received said that my site had come up in a survey they did asking teachers for blogs they read. Assuming they were not just making that up to flatter me (always a possibility) I would welcome comments on the WetPaint service from any of my readers who have used it.