Anyway, if you are a user on a network get the Administrator to reset it for you.
If you are the administrator, simply type 'dogsbollocks' three times into the password box of your Log In window. Unless your password happens to be 'dogsbollocks' this should fail each time. You will then be offered to reset your password by using your master password (N.B. if FileVault is enabled this option won't work).
If you don't know your master password, reboot using the MacOSX install DVD and once you get to Installer, choose Reset Password from the menu bar. Select your account and type in a new password.