American Red Cross - Donation Detector
I love this idea from the Miami Ad School for the American Red Cross. Encouraging donations is an extremely hard job, and what better way to do it than when you have to take out your change anyway?
It reminds me a bit of the Danish donation system at supermarkets' bottle return systems. There, once you have returned all your bottles, you have a choice of either printing out a recipet which you can cash in at the check out, or you can press the other button which donates the money to charity. Really quite a good idea.