Forgot Admin password

Insert the installer disc 1 (grey original disc), and start with the C key held down after the chime. When the installer appears, OK the language page, then ignore the installer and go to the menu bar at the top of the screen.

I think it's the Utilities menu, but you can try the others if it isn't - select the password reset (might be password manager) option and follow the instructions.

Then quit the installer and reboot normally.

(That's for all OSs up to Snow Leopard - if you're on Lion, someone else will need to help you)

Earlier than Lion:

Mac OS X: Changing or resetting an account password

Forgot Mac Password? How to Reset Your Mac Password (with or without CD)

Lion:

OS X Lion: Apple ID can be used to reset your user account password

Reset a Mac OS X 10.7 Lion Password

Change the Password in Mac OS X 10.7 Lion Without Knowing the Current Password

If you don't have the install DVD's see retrieve lost password mac os x

If you do have the disks see Mac OS X: Changing or resetting an account password