A few weeks ago my sister sold her computer on craigslist. I had warned her she needs to fully wipe the hard drive before she actually sold it and suggested using GDisk which is what I am familiar with. But she couldn't get access to the program and just fdisk'd it and formatted and and hoped for the best. I ran across
this post which is a free utility that will essentially do the same thing for you. You just download it, burn it to cd and boot, the options are pretty simple, just type in autonuke and it will wipe the drive three times. Thats enough to keep anyone that is only mildly interested in what might have been on the drive out.