A wiki ( Hawaiian term for "quick") is an excellent tool for collaboration in an online environment. It is a collaborative site that allows anyone to contribute to it. Wikis being very open, they provide various means to verify the validity of any additions to the body of pages.
Inspite of its openness, the pages are usually balanced and provide comprehensive information -- there is hardly any disorder or confusion. Visit Wikipedia, the free encyclopedia, to see how a wiki works -- it is one of my favorite resources.
Wikis are now even looked upon as useful tools for facilitating online education. Read this excellent paper Teaching and learning online with wikis by Naomi Augar, Ruth Raitman and Wanlei Zhou. The paper outlines basic wiki functionality and highlights the features that make wikis a valuable technology for teaching and learning online.