When you can't melt it or crush it.
How often do these scenarios come up?
- You want to donate a home PC that is a few years old to a charity, but you've processed your income taxes and other personal sensitive information on it.
- You have to return a computer furnished for a project by a consulting customer at the end of a contract. You had to run/develop proprietary software on that machine that is not included in the end deliverables.
- You have a new computer that you want to sell after reloading the OS, but you want to make sure that any personal information processed on that machine cannot be accessed later.
One simple choice for the x86 crowd is Darik's Boot and Nuke aka DBAN. This bootable Linux CD is distributed in an ISO file. So all you need to do is burn it to a CD, boot it up, and trash all your data. While the algorithms are very effective, if you need 100% risk avoidance, the best option is still pulling the drives out and smelting them into slag.
DBAN is totally free, but if you have a business need that requires U.S. DoD 5220.22-M, Sarbanes Oxley, HIPAA, or FISMA compliance, you might want to try the supported version called EBAN, or Enterprise Boot and Nuke.
My old light weight favorite, Autoclave, was discontinued by the University of Washington in 2002, but still works really well on older machines (i.e. pre-Y2K) you may have that can only boot from a floppy disk drive.