@ Those still obsessed with hammers.
Are you really trying to protect your data from the NSA and GCHQ? Do you really think that they want the information on your disks?
If not, then read some of the comments about wiping and DoD standards above. You could also try watching the BBC report about the Which report in which a spokesman for a data recovery company states that a drive overwritten with zeros can't be recovered.
Could anyone who is advocating hammers please point me to one source that states that data has been recovered from a drive overwritten with zeros?
I'm not some bleeding-heart "charidy" type person or a "Carbons" head but not giving hard drives to charity or selling them because a guy once said it /might/ be possible to recover data from hard drives that have been overwritten with zeros is paranoia bordering on clinical.