How to Securely Wipe a Hard Drive - Darik's Boot and Nuke ("DBAN")

Darik's Boot and Nuke ("DBAN") is a self-contained boot disk that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.

DBAN is a means of ensuring due diligence in computer recycling, a way of preventing identity theft if you want to sell a computer, and a good way to totally clean a Microsoft Windows installation of viruses and spyware. DBAN prevents or thoroughly hinders all known techniques of hard disk forensic analysis.

DBAN is a free software product that can be used at home or in a business at zero cost.

DBAN has all available drivers for SCSI disks and drivers for IDE, PATA, and SATA disks.
DBAN supports all Microsoft platforms and securely destroys FAT, VFAT, and NTFS filesytems and  all unix platforms and securely destroys ReiserFS, EXT2, EXT3, EXT4, and UFS filesystems.

Installation:
DBAN 2.2.6 Beta for CD-R and DVD-R media.
(Burn this file to a blank disc and boot the computer with it. Do not unzip this file.)
Burn this downloaded ISO file to a CD and boot off that CD (you need to configure your BISO settings to boot from the CD first). Follow the prompts and select your appropriate configuration options.

DBAN is easy to use and very secure, using multiple methods including the Gutmann method and the Mersenne twister.

The autonuke option from the first screen will automatically run through all detected disks, securely wiping them, so be careful with it.