Secure deletion: a single overwrite will do it

The myth that to delete data really securely from a hard disk you have to overwrite it many times, using different patterns, has persisted for decades, despite the fact that even firms specialising in data recovery, openly admit that if a hard disk is overwritten with zeros just once, all of its data is irretrievably lost. Even one wipe with zeroes would require that someone disassemble the drive and use specialized equipment to recover the data... hard drives leave traces that are a little wider than the tracks, so special super-sensitive probes can read the edges and recover the original data. No garden-variety garage hacker is going to have that stuff; it costs a mint.

The 7x and 35x overwrites are meant to make it very difficult even for the specialized tools to recover the data. Overwriting with zeroes is predictable, so the weak fluxes in the track edges can be reassembled into data, but when random data has been repeatedly written, it should be extremely difficult to sort out which bits are correct and which are just noise. As long as nobody risks jail time if the drive falls into the wrong hands, even zeroes is probably enough, but 7x will give you a very high degree of safety. 35x is for the ridiculously paranoid.

dban
Darik's Boot and Nuke ("DBAN") is a self-contained boot floppy that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.

DP Shredder
DP Shredder scrubs files, folders, and free disk space with Department-of-Defense-approved methods. Completely stand-alone and small, DP Shredder is a great addition to your flash drive toolbox. After selecting the disk, folder, or file you want to securely delete with DP Shredder, you can select the method and number of times the method will be applied. You can use a basic overwrite with all zeros, pseudo-random blocks of data, or opt to use more robust methods. DP Shredder can wipe with the power of US DoD 5220.22.M ECE 7X—which, if you must know, deletes the files, overwrite all locations with a character, its complement, then a random character followed by an overwrite, and it's repeated seven times. At the end of a single round, every bit of data you've deleted has been over-written 49 times. If you need more assurance that your data is gone, it's time to break out the drill and find a far-away landfill or e-cycling centre. DP Shredder is freeware, Windows only.