Darik's Boot and Nuke is a bootable tool - in ISO format - which securely wipes the hard drive of your choice. It's ideal if you're about to sell or scrap a PC and want to be sure no-one can recover your personal data.
After burning the image to disc (right-click, "Burn disc image" in Windows 7/ 8), boot your system from it. Darik's Boot and Nuke has a text-based interface and so doesn't exactly look pretty, but read the prompts and it's reasonably easy to use.
If you want to wipe every drive connected to the system, for example, just type autonuke when prompted, press Enter, and wait as the program overwrites your data. (There's no "are you sure?" check, so use this with care.)
If you need more control, press Enter to launch Darik's Boot and Nuke's "interactive mode". Here you can configure details like the wipe method (Quick Erase, RCMP TSSIT OPS-II, DoD Short, DoD 5220-22-M, Gutmann Wipe, PRNG Stream), number of passes, the drive to be wiped, whether you want to verify your data's destruction, and more.
When you're happy with the settings, press F10 to start the overwrite process. As before, the program launches immediately, with no "are you sure?" check: don't touch the keyboard until you're entirely sure what you're doing.