To reset the administrator password using the Mac OS X disc:
Insert the Mac OS X Install disc and restart the computer.
When you hear the startup tone, hold down the C key until you see the spinning gear.
When the Installer appears, choose Utilities > Reset Password.
Follow the onscreen instructions to change the password.
Quit the Installer and restart your computer while holding down the mouse button to eject the disc.

Actually most, if not all, OS X bootable install disks had a
password erasing capablity built into them. I know for
sure that OS X 10.2 through 10.4 has it.

Find one of these disks, place it in the optical drive,
restart the computer and when you hear the boot tone,
hold the 'C' key down. This will start the machine booted
from the CD rather than the internal HD.

If this doesn't work, hold down the 'option' key while
booting and when the blue screen appears select the CD
once the hourglass has gone away and then select the
proceed button that looks like an arrow pointing right.

Once booted from the CD next to the Install OS Xb> name in the top left side of the menu bar should be a
'Utilities' menu or something like that. There should be a
reset password option in one of those menus.

HOW TO: Reset a lost OS X password in single user mode

I've you've forgotten your Mac's admin account password, don't worry. Assuming you haven't locked out OpenFirmware, it's a pretty simple task to change your password back to something you know.

Here's how:
Hold Apple+S when booting to enter single user mode

#sh /etc/rc
#passwd yourusername
#reboot

If you can't recall your user name, you can either look in the /Users folder (the directories are named by user), or run "niutil -list . /users".

Also, on older systems the /etc/rc script isn't available, apparently. If that second step fails, try mounting and starting the base services manually:

#/sbin/fsck -y
#/sbin/mount -uw /
#/sbin/SystemStarter