Google Now gets you just the right information at just the right time.
It tells you today’s weather before you start your day, how much traffic to expect before you leave for work, when the next train will arrive as you’re standing on the platform, or your favorite team's score while they’re playing. And the best part? All of this happens automatically. Cards appear throughout the day at the moment you need them.
Of all the announcements Google made today, the most useful is Google Now. Check out the video in the link above. If done right this will be the most useful App/Service, Google has produced so far outside of Google Search. Ofcourse it will come with a lot of Privacy implications and Google does not do well in this aspect, atleast not so far.