DBAN
Another great free and Open Source utility is DBAN – Darik’s Boot and Nuke. This is a great bootable CD/Floppy that securly erases disks. Ever want to sell or get rid of a hard disk or PC? This lets you be sure that there is no data on the disk. It boots up straight from the floppy and will securly erase all data on a hard disk.
You can download it from http://dban.sourceforge.net/