In OS versions as early as 10.3 (and possibly later incarnations of 10.2.x), there was an application on the CD installer disc that enabled you to change your password. This was still the standard method to reset passwords on Macs through 10.6. With the release of machines running Lion and subsequently Mountain Lion, Apple no longer includes restore media as part of the accessory kit. How do we now reset passwords?
In Lion, 10.7.x and Mountain Lion, 10.8.x, the password reset is not an obvious utility when booted from the Restore partition of the drive, but the functionality still exists on these machines. The same password reset utility is available on your Mac via the Restore partition and it does not require a lot of digging.
When booted from the restore image, do not select any of the default options (Safari, Disk Utility or Restore), but rather, click on the Utilities option in the menu bar and select Terminal. In Terminal, simply type passwordreset at the prompt and the familiar password reset application found in 10.2.x through 10.6.x install media will launch and provide the same support as what existed before.
The same caveats still apply—you will have to create new keychains for your account. Accounts using FileVault will still be completely inaccessible as they need the original password to open the encrypted user folder image.