I like this question.

On FreeBSD, I'd do the following:

1. boot to single user mode and enter the shell. By this, you become root!
2. mount the root partition
3. change the password for any user I like.

For Linux, I guess you can boot off the install media (CD/DVD) and enter some recovery console. It's called emergency something....something... I don't quite remember well but it's an option on the install media. Just "open" your eyes when booting off the CD/DVD ;-)