Forgetting the root password on Mac can feel like a disaster but you can easily recover it using a sudo command if you have the password to an administrator's account on the system.

Open a terminal window and type:

sudo passwd root

You will be prompted for your password (the password to the administrator's account). After that you will be prompted for the new root password twice.

The sudo command is a great tool to control who can do what on a system. In this scenario the sudo command allows you after authentication with the user password to run a command as if you were root. The command passwd root is the command to change the password for the root user.

Update 01/16/08 How to reset lost root password by switching to single-user mode (Command-S) known as command line interface in Unix. (Suggested in user comments.)

Reed more on how to change your lost root password with an Install Disk.