If you are passing the drive on, the only free sure way is don't.
Depends if you just want to delete a few files and keep the drive as is for your own use.
If you want to sell or give it away, you can use something like Gparted to write zeros to the hard drive, but with the know how, the right tools and determination it can still be recovered.
It's the same old story, locks only keep honest people out.
You could try Erazer: http://eraser.heidi.ie/