Business card holder. You can put on on your door each day depending on how you feel. So people can go up to your door, and open the box before they open the door, so they know what to expect. Or you could stuff a dead mouse in one and send it to someone who likes altoids. Or you could paint an Attrition.org logo on them and give them out at DefCon.
Tin cans make excellent flower habitat.
Individually, they can provide an armor plated environment for one small plant. If you lash several of them together with a couple of turns of duck tape you can create a flower cluster, a knot of flowers as it were. By strategically placing some cans in an inverted position to be used as spacers, you can control the shape of the cluster. If the above principle is carried to an extreme and enclosed in a large box made entirely of cans except for a plate of glass at the top and eastern exposure, a tin can greenhouse will result, a mini-rain forest if you will. I think you should just bury them in your backyard beside the 4,000,893 AOL cds and the remains of the unlucky Jehovah's Witnesses.