Well, this is probably one of the odder requests I've answered lately.

There are numerous ways you can clean off a penny. The best way is to soak it in CLR, Lime Away, or similar metal cleaning products. I think things like bleach and Oxy-clean work as well.

If you don't have any of those, the easiest way I know of is to soak the penny in soda. Pepsi, Coke, or whatever you've got. As long as it's still fizzing, it's good. The carbonation helps dissolve the dirt and lime deposits on the penny's surface. It won't get quite as clean as it would with cleaning products, but soda is cheaper and easier to come by.