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Wikispaces are offering free wikis to teachers, with no adverts and the ability to restrict it to private groups, normal cost $5. Go to http://www.wikispaces.com/site/for/teachers100K for more details.
Wikipedia describes a wiki as ‘a website that allows the visitors themselves to easily add, remove, and otherwise edit and change available content, typically without the need for registration. This ease of interaction and operation makes a wiki an effective tool for mass collaborative authoring.’  
Wikipedia (http://en.wikipedia.org/ ) itself is an excellent example of this, being a free online encyclopedia which can be edited and added to by any user. 
Another example can be found at http://shropshireict.wikispaces.com/  
Schools, classes or group can create their own wikis in order to share, develop and edit information collaboratively.