need to make sure data is gone ...

Hello all, I am in the process of moving and I want to get rid of (sell, donate, give away) all of my old spare parts and self made boxes. I have about 20 hard drives of various sizes and I need to get rid of them as well. My question is, how can I make absolutely sure that all the data has been erased from those drives before I give them away. The drives all came from clients that I did upgrades for and they did not want to keep the old systems. I have no idea which drives have sensitive information on them and which ones do not. Thanks for any suggestions in advance. -- d33p th1nk3r