Single-user mode

In single-user mode, there is and can be only one login to the system, which will be the root user. This mode is needed if you want to unmount crucial partitions like /usr or /var, ie for fsck-ing purposes. (You can fsck a mounted partition, but you CAN'T actually repair it while it's mounted - to repair it, you have to unmount it first and THEN fsck it.)

One way to drop to single-user mode is by typing init 1 as root. WARNING: you'd better be physically logged in at a console when you do this, not shelled in across the network, because when you drop to single-user mode on a box, that box's network capability goes AWAY.

Also, the second CD in the FreeBSD set is a FixIt disk that boots you into single-user mode.