Oh, yes! I like to call it the "TOTALLY SELFISH ACTIVITY!!!!" I learned this one from Laura Brandt at the AP Conference in DC 2010.

Points can vary up to the teacher in terms of how many extra credit points you give. 5 or 10 or your choice

On a scrap piece of paper, students write 5 or 10 write name for extra credit purposes, but not to ever be revealed (anonymity)
THERE is no talking, so students cannot collude

*IF the whole class writes "5" everyone gets 5 extra credit points-WIN WIN FOR ALL! VERY rarely happens...someone in room is selfish
*If one-three people write "10" ONLY THOSE PPL. GET extra credit - the rest of class gets NONE! These students also have incentive of 10 points, not just 5! (I've actually had students do this; I tell them it is anonymous - no one will ever know! There are still students who want to know who those "10ers" were from last year) - this is a WIN for the "10ers" and LOSE for those who wrote 5 *If FOUR or more people write "10" and get selfish: NO ONE IN THE CLASS GETS POINTS (Lose-lose situation)

They never forget the activity - it drives them CrAzY!