vtgrab is a utility for monitoring the screen of another machine. It only works for text consoles: for X, the idea is to use VNC instead.
You can use it like this:
# ssh observed 'vtgrab --server' | vtgrab --client
(Unfortunately, it needs to run as root on _both_ machines.)
The eventual aim is to be able to use this in a teaching environment in such a way that whatever is on the student's screen can be seen by the tutor, including graphics. Ideally, someone would write an x0rfbserver that would help with that last bit, hint hint Jens.
There seem to be some bugs which I don't fully understand in this release: vtgrab sometimes has bad interactions with X when switching the foreground console. It might be something to do with VT_RELDISP.