nca connects a pseudo terminal to the console's virtual terminals (/dev/tty0...n) that are normally only accessible to someone with local keyboard access.
It is particularly useful in conjunction with ncad(8) since it then provides remote console access via SSH; with some appropriate system start up scripts, ncad can be started immediately the root filesystem has been mounted read-only, providing a cheap, and only marginally inferior alternative to a serial console.
Suppose, for example, that you installed ncad on one of your servers, and arranged for it to be started immediately the root filesystem has been mounted. The ncad start script brings up a IP address, and sets up some basic networking, so you are able to log on the remote server and watch it boot up. Should problems occur (filesystems require repairing, perhaps) you can break out into a shell and fix them. You can even arrange to have ncad started in single user mode, allowing you to conduct low level maintenance (moving or resizing partitions for example) remotely.
See README file inside the tarball.
See README.patches for details of the patches to sshd necessary to allow it to run so early in the boot sequence.
nca is maintained using GNU Arch. You'll find a mirror of the repository at http://arch.xenoclast.org/phaedrus-2004.
I'm currently working off the ncakit--dev--1.0 branch, so the following will get you the latest version:
tla register-archive http://arch.xenoclast.org/phaedrus-2004 tla get firstname.lastname@example.org/ncakit--dev--1.0 nca
nca was written by James McKenzie (email@example.com) and documented and packaged by Julian T. J. Midgley (firstname.lastname@example.org)