Keylogger.

If you want to know what other users are typing on your machine, this heavy weaponry addition to your Spy Arsenal will allow you to know everything! Simply install Family Key Logger, set the 'Start in hidden mode' and 'Remove from tasks list' options and you are ready to go. The program will reside in memory while being absolutely invisible to everyone except you. You will only have to press the Unhide key combination on the keyboard to see what your kids or your wife or your employees were typing.

And remember:

You have the right to know!

Source: http://www.spyarsenal.com/