I use either CBL Disk Shredder or DBAN software, depending on the age/drive support on the system, to erase the drives. Then if it likely didn't have anything very interesting, it can go to a recycler. If it had critical data I take it to the next paranoid level and remove the platters, grinding, cutting, and bending them up if they're metal. If ceramic I wrap in a paper towel and give them a slight bend with pliers and they explode (hence the paper towel, otherwise you get jagged ceramic shards everywhere) and the whole thing gets dumped into the trash.