ncp is a suite of programs to quickly copy files between machines.

It is especially tailored to LAN use, but can be used over WANs, too.
I recommend against that, though, because the protocol is really trivial
and does not include recovery or error checking, it just does a tar over
a TCP connection.

The theory of operation is that two friends sit in a LAN on their
computers and one wants to copy a file to the other.  Than one of them
runs

  $ npoll

and waits, the other one runs

  $ npush the files he wants to copy

This will work if they are on the same LAN segment, because npush will
broadcast UDP packets on port 8002 to all machines on the segment
announcing its willingness to send something.  npoll just sits there and
waits for these packets to appear and then opens a TCP connection to
port 8002 on the sending machine.

[8002 has been chosen arbitrarily and is 8002 for historical reasons]

The UDP broadcasts are sent every 0.2 seconds, and npush has provisions
to answer to udp broadcasts from npoll, so npoll could speed things up
by sending packets, too, and then npush would answer immediately, but
that is not implemented right now.
