Sliced bread?
Cause you always hear that something is the greatest invention since sliced bread.
All kidding aside after some thought I'd say Computers.
Computers have allowed the the planet to grow in new ways. I can go to a bank branch 1000 miles away from home and they can pull up my account information instantaneously. I can go in the supermarket and can get checked out right away without having to wait for someone to check the price on a shelf because an item was missing a price tag. I can play games in virtual environments that are approaching the quality of a movie.
I may not be giving the best examples but I think you get the idea.