Format the hard drive and reinstall the OS. You'll obviously need the OS DVD to do this. If you are running Snow Leopard or earlier I'm pretty sure they have an "Erase and Install option." After you do this, if you are really paranoid, you can use disk utility to "erase free space" which writes zeroes to the free space to ensure your data can't be recovered. The 7-pass option is probably enough.
They do have the erase and install option and you might have to click on options to see that. You have to boot from the DVD to do this installation and keep in mind that zeroing out the hard drive takes quite a bit of time, the bigger the drive the longer it takes, so don't try to do it on your way to work.
If you have nothing to be paranoid about, you can just put the DVD in, boot it up and select the option to erase and install. That will put it back to "brand new" as far as the average user is concerned. And don't put in any personal info for the setup process when it finishes the installation.
If you don't have the original DVD, you might have to buy one that is a full install. You can't use one that came with another machine.
If the machine happens to have another user account that you can get into, you can probably just delete your user account and that will get rid of all your mail, docs, photos and personal stuff, but not applications you installed.