It depends on what you mean by piece of technology I guess, since what we have now is a culmination of thousands of awesome tech over the last few hundred years.
If I were to choose one thing, I'd say the telephone. It's the predecessor to the internet, and suddenly communication between people was instant rather than messages that'd take forever (or morse for the places that had it).
It probably changed the world forever, being able to talk to someone in a completely different country and share something quickly.