Most of my experience with wikis has to do with Wikipedia. I have used it extensively to research all different topics. In a music classroom, students could use a wiki to plan and organize a talent show. They could each submit songs or acts for approval by the teacher and check back to see if their ideas were approved. Then the teacher could put the acts in order and the students would have an order for the program. The students could then help to create a handout for the talent show on the wiki.
Between teachers a wiki would be a very useful tool for sharing ideas, posting links to helpful websites, organizing a choir festival, etc. I get e-mails from teachers all year long with links to helpful websites. Once the e-mail is buried under all my other e-mail, I can't find it any more. If teachers were sharing websites on a wiki or a site like delicious or diigo then they would be easy to find.