If you're selling or giving away your computer, hard drive or other storage device, make sure that any sensitive or personal data has been securely erased. A single reformatting pass is far better than nothing, and if you are in a hurry, a single-pass reformat that zeroes out your data is even better.
But for a secure reformat, you'll want to go with a 7-pass reformat that writes data over your disk 7 times; this of course may take quite a while. For the best security, you'll want to go with a 35-pass reformat, which can take many, many hours to complete depending on the speed of your processor and the size of your drive. The 35-pass method is used by institutions that handle large quantities of sensitive financial and identity data. If you've been keeping your tax records on your computer and you're about to sell it on Ebay, I'd certainly go with a 7-pass reformat.
People who are using Macs with OS 10.4 (or later) installed -- or who have access to 10.4 installer DVDs -- will find this process to be a breeze, since secure erasure methods are built into the Disk Utility. The MacOS Disk Utility can format or reformat UNIX, Macintosh, and Windows operating systems, so if you have a friend with a modern Mac and a firewire or USB external drive, you might ask to borrow their Mac for a little while. The Mac Disk Utility also lets you securely erase free disk space in instances where you want to pass along a used Zip disk, external hard drive, etc. with audio or video files intact but everything else inaccessible. It's also handy when you've prepped an old computer for someone else with a fresh copy of the operating system and select programs, only to remember that, oops, you forgot to do a secure reformat before you spent 3 hours on reinstallations.
Windows and Linux users who don't want to deal with a Mac have other programs they can use for secure data erasure. One piece of free software that many techs prefer is Darik's Boot and Nuke ("DBAN"), which can be downloaded at http://dban.sourceforge.net/. It will work on fairly old hardware, and the only desktop operating systems it doesn't support are Amiga and MacOS.