This post is intended for users that forgot the root passwords or can't use it due to other problems (corrupted file /etc/passwd or whatever). Can also be used to gain unauthorized to linux machines, but don't do that.
What to do Boot using a recovery CD or DVD.
Mount the drive
mount /mnt /dev/sda2
Replace the existing root password with an empty one
sed /^root/s/.*:root:/root::0:0:root:/ /mnt/etc/passwd
And reboot - and you are done ;-)