Because of the blocking in my district, I've been unable to use Wikis. (Ok - I did last year in a "pilot project.") The project last year was having students collaborate with a school in Alabama. Students in both states learned the same material and used a wiki to share learning. They edited each other's posts and I was surprised to see 7th grade students edit spelling and grammar!
I hope to use a wiki this year (pilot still to be approved) to have students complete an online "study guide". I plan to outline the wiki study guide with requirements from our state core. Students will fill in examples and descriptions of what these requirements look like in life. I would like to make student participation worth 5-10% of an examination of that material. The ability to review student work and edits would make this fairly simple to accomplish (IMHO).