Yes Home Depot does accept old propane tanks for recycling and free of charge.
Just bring your old propane tanks to a Home Depot Store and let someone know that you have some Propane Tanks for recycling and they'll assist you and let you know where you drop the old propane tanks off at.
Home depot also accepts cell phones, nickel cadmium rechargeable batteries and compact fluorescent lights for recycling.
You can also use your old propane tank for an exchange for a full propane tank at Home Depot or any other store that does Propane Tank exchanges.