Free, open-source boot disk utility Darik's Boot and Nuke (DBAN) automatically and completely deletes the content of every hard disk it can find on your computer when you run it. Sure you can fire up DBAN for emergency system wipes next time the feds come knocking on your door, but it's also a useful tool for protecting yourself from identity theft when you're prepping your computer for recycling or sale. The bootable DBAN can run from CDs, DVDs, thumb drives, and floppy disks. If you're just looking to securely delete single files and folders (as opposed to entire drives), check out previously mentioned Wipe File.