Darik's Boot and Nuke toolDarik's Boot And Nuke (DBAN) is one of the most popular tools that can be used to wipe a hard drive of all contents. For drives that have contained sensitive data, this is a critical step if the drive is to be sold, returned, or otherwise leaves your possession.
DBAN is a free tool and is distributed as a CD-ROM .ISO image. Download the .ISO image from the DBAN website and write it to an optical disk.
(Click on images to enlarge.)