So I'm selling off an old MacBook and I need to wipe its hard drive - the problem is that I don't have my Installation Disks anymore. So I need it to be wiped clean, but I'm not sure how to do so without the aforementioned disks.
You can use the Erase Free Space feature while booted into the system itself.
First, delete your user files: I recommend that you create a new, admin user, log in as that user, and then delete your previous user(s) completely using the Accounts preference panel. (Choose to "Delete immediately", not save as an image). Also uninstall any applications that you aren't passing on the disks or licences for.
Then, using Disk Utility, click on your HD on the left side, click the Erase tab, and use Erase Free Space to write over the now-empty space on your drive. One pass of zeros is usually quite sufficient (unless you have military secrets).
While Matt's solution clears most user stuff off the machine, I strongly recommend that you contact Apple for replacement discs (should be available for a nominal S&H charge), since buying a used computer w/o its original discs is like buying an used auto w/o its spare tire and jack. I usually advise purchasers to pass on such an item. Once you get the discs, boot with the restore one and zero out the data and shut down the machine. Let the new user restore everything to factory specs.