It blew my mind when I wondered how many femtoseconds it had been since the start of the Roman Empire, and it obligingly told me: http://www.wolframalpha.com/input/?i=now+-+founding+of+the+r....

It also told me how many ergs are in a jellybean and what time the sun would set on April 2nd (http://www.wolframalpha.com/input/?i=sunset+april+2nd%2C+201...).

Of the many technologies I use, Wolfram Alpha is the one that feels the most like the future sometimes.