Here are some ways I used wikis with my students:
1. Third grade students are part of a folksonomy project where they are coming up with their own tags for books. Often, students search the OPAC for "funny books" and hardly any results are displayed. Students brainstormed tags and use a wiki to add titles as they see necessary. Here it is: http://fosterbooklists.wikispaces.com/
2. Fifth grade students found their favorite author and illustrator web sites and added them to this page: http://fosterlibrary.wikispaces.com/
3. Teachers request books and AV equipment on a wiki too: http://fosterav.wikispaces.com/
(I made all three wikis public for the time being, but will reset the privacy options in a week or so).
P.S. I also posted the tagging project on the PA Curriculum wiki.