I encourage that you save your work and if possible make a backup prior to performing this procedure.
Note: I have confirmed this to work on Mac OS X 10.4. Please comment if you have been able to use this in other versions.
Firstly, make note of the username(s) you wish to reset. This can be found in System Preferences. You will need the Unix username which will not have any spaces. For example, if your Display Name on the Mac is John Smith, your username is likely to be johnsmith as configured by the Mac. Preferably make note of the main administrator account. Once you have reset this password, you can make other changes later.
Once you have made note of the username, reboot the computer.
On startup (about the time of the startup sound), press and hold Command+S
Once the system has booted in the Safe Mode or Single-User Mode, you need to enter a particular command shell. Enter the following:
then press Enter.
You will now be able to reset any user account on this PC using the following command:
After the passwd, enter the unix username for the user account that you wish to reset. You will be asked to enter the new password in twice to confirm the change.
Once the changes have been made, you can now reboot the system. Commands reboot or init 6 should initiate the reboot.
Remember, please leave a comment of your experiences below.