Do I even have to ask if you've heard of Wikipedia? It always seems to be the first hit when you search for a general topic. Wikipedia = wiki = collaborative websites. Anyone can add to Wikipedia, edit Wikipedia or even remove information from Wikipedia. This is my father's page that I helped to create.
The same features are available - for free - at a number of wiki sites, some of which have been set up for education. These include pbworks.com and wikispaces.com.
Pbworks offers tutorials to help you learn the ins and outs of using their wiki. So does Wikispaces. I have several wikis in operation, including tbycomputers.pbworks, tbyart.pbworks and tbyresources.pbworks. You are welcome to poke around these wikis, and even edit them, however some pages are private and you'll need to request access.
So how can you use this with your class? Let's use a geography class as an example. Each group is assigned an area of Toronto to research. Students can then create one or more pages on which they put a map, demographic statistics, images, movies, tables and more. Geographyproject.pbworks.com is a basic example. They can do this in school, in our computer lab, or they can do it individually from home. And you, as the teacher, can tell what each student has done, because a history log attached to each page tells you the login name of the student and exactly what they have contributed. Students (and teachers) can also leave comments on each page - a feedback system.