If you set a firmware password, remove it by running Firmware Password Utility in Recovery mode.
If you activated FileVault in OS X 10.7 or later, turn it off.
If you use Boot Camp, the partition must be deleted.
If you created any other data partitions on the internal drive, remove them in Disk Utility.
Erase the data partition(s) with the option to zero out data. An SSD doesn't need to be zeroed.
You can't legally or practically transfer any software downloaded from the Mac App Store to the new owner of the machine, even if it was free. That includes OS X, so if you upgraded to OS X 10.7 or later, you must reinstall an older OS, either from the installation media, if applicable, or by starting up in Internet Recovery mode (option-command-R at the startup chime.) If you installed from physical media, deliver those to the new owner.
If you're selling the machine, or donating it in working order, and it originally shipped with OS X 10.4 or 10.5, then you have the option of installing either from the discs that came with it or from a retail Snow Leopard disc (which you must then transfer with the machine.) The buyer should understand that if he doesn't get the original discs from you, he won't get the bundled iLife applications or the Apple Hardware Test. Replacements for the original discs can be ordered from Apple.
The new owner will have to redownload any software that came from the App Store, including OS X upgrades, under his or her Apple ID. If you ever updated the bundled iLife applications (Garage Band, iMovie, and iPhoto) through the App Store, you can't transfer those either.
Remove the machine from your list of registered products. If it's still covered by an AppleCare Protection Plan, transfer the coverage to the new owner by following the instructions in the AppleCare Terms and Conditions (under the heading "Transfer of Plan.")