When I was a kid, I made firestarters for my dad as gifts for special occasions with my old crayons. My parents have a wood burning stove and he loved them.
There is a similar program called the Crayon Initiative which does the same thing but I don't think you mail them anywhere. Rather, local restaurants collect the used crayons that they give out with their kid's menus. (most restaurants in my area give a brand new small pack of cheap crayons with each kid's coloring menu)