Q:

Hidden (or not commonly known) windows and unix commands?

What is that one not usually known command in unix and windows that you know? 
It is heard that windows contains several hidden applications which sometimes
may be very useful.

A:

linux:
history (history of command line)
mogrify (for all image needs/operations)
screen (for running programs after logging off via ssh)

A:

In widows XP if you have ever tried to do somthing like this
cd \\pc\c$

You will have recieved the error
CMD does not support UNC paths as current directories.

Well you can use UNC paths as long as you map them to a temp drive letter like so.
pushd \\pc\c$

Then when you want to return simply...
popd

A:

Windows:
fdisk /mbr

Saved my life (and system) after a Linux partition went berserk.
Linux:
strace

Came handy getting passwords with classmates running a telnet from a shell I was logged in ;-)

