Step 1: Get a FireWire Cable
Both of the Macs will need to have FireWire, which means everything between the iMac DV and the current lineup (except for the MacBook Air and unibody MacBook) is eligible. You will also need a FireWire 400 cable so you can connect one end to the old Mac, and the other end to another Mac. If you don't have one of these cables, you can easily pick up one online or at a retail store. If both of the Macs support FireWire 800, you can use one of those cables as an alternative to 400; it will be faster. If one has only a 400 port and the other has only an 800 port, a converter cable (available online for less than $10) is what you need. Once you have the FireWire cable, just simply connect the two computers together.
Step 2: Boot into Target Disk Mode on the old Mac
Shut down the old Mac that will be receiving the hard drive cleaning, and reboot it while holding down the "T" key on your keyboard. After a few moments, you should see a FireWire symbol appear on the screen -- when you see this, you are in FireWire Disk Mode (FDM). When in FDM, your old Mac's hard drive should show up on the new Mac as a connected external FireWire drive.
Step 3: Do some Disk Utility magic
Open Disk Utility.app on your new Mac (located in /Applications/Utilities), and click on your old Mac hard drive in the selection area on the left. Note that you should click on the drive, and not the partition (often called "Macintosh HD") to ensure a complete disk wipe. The disk drive will most likely have a FireWire icon to denote that it is connected as an external disk. Once you select the drive, click the Erase tab, and click Security options.
In this section of Disk Utility, you will be able to select a few different erase options that will also add security to the standard disk erase. First, lets specify a name for the drive -- in the name field type what you would like the drive to show up as when it is erased, otherwise it will default to "Untitled."
Continue reading to learn about security options and how to fully erase the hard drive on an old Mac.
If you click the "Security Options..." button next to Erase you will be presented with a few options that can add security to a standard erase. By default, Disk Utility erases the disk directory instead of the data on the drive, meaning that a recovery utility could bring back your information. This is the least secure method, and isn't appropriate if you have sensitive items on the old drive. I recommend that you do a zero out or 7-pass erase. If you select 'zero out data,' your hard drive will have 0s written in place of all the information stored on the disk. If you want more security than that, you can select the 7-pass erase; with this method of erasing, the drive will be fully erased 7 times. This method is the same security measure that the United States Department of Defense recommends to securely erase magnetic media.
Once you select your method, click OK, and then click on the erase button. Depending on the erase method you selected and the size of the old drive, this process could take from a few minutes to several hours -- for a 7-pass erase on a large drive, I'd suggest starting it and letting it run overnight. Once it is erased, you will see the drive show back up in Disk Utility with the new name that you specified. At this time you can eject the disk from your new Mac and turn off the old Mac. Your old machine has been completely erased and you can rest assured that your important information will not get into the wrong hands. Of course, the old machine will need an OS install before it can be used, but that's the next person's problem.
For more tips and tricks like this, visit the TUAW Tips section of TUAW.