Good free disk data destroyer

Any time you give a computer to charity or dispose of it in any way
for that matter, you need to wipe the hard disk(s). The best free software for this pupose that I have found is Killdisk.

I couldn't explain any better than they do at the killdir.com website:

"This is security software for unrecoverable data elimination for any computer capable of booting in DOS mode from floppy drive. It uses access to the drive’s data on a physical level via BIOS bypassing logical drive structure organization, thus it does not matter what operating systems and file systems located on the machine.

It can work with DOS, Windows 95 / 98, Windows NT / 2000 / XP / XP Professional x64 / XP Home x64/ 2003 Server x64, Linux, Unix for PC."

Do I need to mention that you need to be very, VERY, VERY sure you have all the data you need off of the drive, first?