Wikis are a cool idea. I liked the idea I read about doing collaborative note taking on a wiki. Students would need to contribute information, make comments, ask questions, give their opinion, make connections... That could be a simple participation grade.
With Model United Nations, students are grouped by the country they represent and by the issue they are researching. They could establish a country wiki which would allow them to share their research more easily and an issue wiki to share information about their issue.
With the information learned about their country, each group could create a trading card with a picture and information about their country to share with the rest of the MUN members at meetings.
I have some projects that students work on as a group, such as a WW II magazine. Each group could establish a wiki as a central place to put information, create and edit their assignment.
In the past, groups have had trouble with students e-mailing information to each other.
It seems like there are similarities between wikis and Google Docs. I'm not sure about that.