Every semester, I reformat my classroom computers to get old junk off and update the applications. A couple semesters ago, I decided to leave iChat (Apple's instant messaging application) active, although I did not put the icon on the desktop, nor did I tell the kids. Our school district has a pretty tough firewall, so I knew that the only chatting possible would be within my classroom network.
In my classes, I have two levels of students: beginners and intermediates. Only the advanced students, the seniors, get a class to themselves. Slowly, they began to find it. It started off predictably enough. There was chitchat about the opposite sex, who's dating who, "Where you going tonight?" Whenever I approached students using iChat, they would quickly hide the window so I wouldn't know. Now, I've been teaching long enough to be able to tell who is on task and who isn't, even if I can't see their screens, so I was well aware of what they were doing.
Then, an interesting thing happened. The beginning kids began asking the intermediate kids for help, and the more advanced kids started tutoring the beginners. Most of the idle chat disappeared, and only this peer-to-peer teaching remained. It was remarkable to see them helping each other.
I never told them I knew what was going on. Why spoil the guilty pleasure of fooling the teacher!