Wikis are a great tool for collaboration. I teach wikis in my information literacy course and, of course, why Wikipedia is not a scholarly tool. Rather than force the students to listen to another teacher rail against the evils of Wikipedia, I use clips from the Colbert Report to do it for me. Colbert does a great interview with Wikipedia founder Jimmy Wales and has several clips where he asks "the nation" to make specific changes to articles, which his fans quickly do. This behavior has resulted in several pages, including elephants, Colbert's own page, oxygen, Einstein, and alpacas, to be locked. These examples show how easily changes can be made to entries. My students then evaluate the Wikipedia article on the topic they have been researching.
Wikis are good for collaboration because they allow the users to work at the same time, from a distance, track changes, and add text, links and images.