Do you have multiple users on your Mac? Sooner or later, somebody will forget their password. If you are the Administrator, you can simply reset it for them.
Here is how:
1. Open 'System Preferences'.
2. Choose 'Accounts'.
3. If the padlock in the lower left is locked, click it to unlock.
4. Authenticate with your Administrator user/pass.
5. Click on the account you want to reset.
6. Click on 'Reset Password'.
7. Enter the new password and verify it. If you want to generate a random one, click on the 'key' and user the password helper. If you use it, make a note of it. You can copy/paste it into the New password and Verify fields.
8. Click the padlock to secure this system preference.