BSD systems have a neat watch tool for snooping other ttys :
Have you ever had a user login to your box and then ask you for help? Or have you ever been curious as to what your users are doing on your system? The watch(8) utility will let you snoop another tty, allowing you to see what your other users are doing and even interact with them. It’s a must-have utility for every network administrator.
watch is a different beast on linux.
So far I've got:
ttysnooprequires modifying /etc/inittab which is a pain
conspycouldn't get it to work so far
peekfdworks but only one way (you can see user input but not output)
screenis nice of course but user needs to use screen ...
Isn't there a good equivalent for linux ?