Say you set up a box and installed your favorite linux distro on it. Say that you wanted to try a spiffy new password that you think you will remember forever. Say you don't see the box for a month and that password has slowly seeped out of your brain. What do you do?

Step 1:
Find you a bootable linux OS (knoppix comes to mind), burn it, and boot it.

When you have finished booting, mount your local drive:

mount /dev/hda1 /mnt/test/
Step 3:
Change the root filesystem to the local hard drive

chroot /mnt/test /bin/bash
Step 4:
Change yourself a password

Step 5:
Shutdown the OS, remove the disk, and reboot.

Welcome back to your system!