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Secret features which enhance the user experience

OK, I have to admit they are not strictly secret like so called Easter Eggs, features hidden in widely used software, which the programmers think are great fun but which some of us think are a waste of our computing resources.  

One notorious example was the Flight Simulator built into Excel97. Microsoft apparently banned the practice in later years as part of its trustworthy and openness initiative but they are still quite common

No, I am talking about very useful features, which many people do not seem to know and which do not appear to be widely publicised.