Darik's Boot and Nuke, or DBAN for short, is open-source software that helps wipe data stored on hard disk drives. It can also work to remove viruses and spyware from Windows installations. The information it removes is not recoverable because it overwrites data with pseudorandom numbers, so the erasure is quite permanent. It's hosted on Sourceforge and states it is "appropriate for bulk or emergency data destruction" and intended "for individuals or home use to support data removal needs." Best of all, DBAN is free and easy to use!