root

If you have executables (programs, scripts), that need root privileges to run (e.g. that mount something), you don't need to type in the su password everytime.
You can set the suid for that file and change its owner to root:
chmod u+s filename chown root filename

If you have forgotten your MySQL root password, you can reset it if you have physical access to the machine mysql is running on:

In (K)ubuntu, only sudo is allowed by default. To enable su, execute:

sudo -s -H

To enable graphical root login, edit /etc/kde3/kdm/kdmrc:

AllowRootLogin=true