I remember an activity I did in school. As I think about it, it was a neat idea. We had to put big 5×7 note cards on our backs and we went around to each other and wrote on their cards about them. Naturally it was anonymous so you could say what you want without being embarrassed, but you also had to write something nice about the person. I think this is a good way to practice having the students get along as well as look for positives about each other instead of constantly trying to one up of take down others. This would be something geared more to the older kids considering you would need them to be able to write, but it could be adapted differently for different age levels.
I think this is something I would want to do in my classroom because I am already seeing the signs of negativity in classrooms starting as young as Kindergarten. Creating a positive classroom environment is just as important as creating one that is academically successful. Not to mention the possible decrease in quarrels and tattle-tales.