The worst thing you can do with old running shoes is toss them in the trash. The next best thing you can with old running shoes is send them to a Reuse-a-Shoe Drop Off Center, where they will be shipped off to a Nike Grind factory, and the foam, rubber and fabric will be reassigned to their next life. How do you find a Reuse-a-Shoe Drop Off Center? Start looking here.
An even better thing you can do with old running shoes is send them to the Perpetual Prosperity Pumps Foundation, which sells used running shoes, and puts the money to good use in African countries. Learn more about the PPPF Africa here.
Another idea is to donate them to a homeless shelter or another charity that collects used clothing.