My immediate reaction is the moveable type printing press. It changed the entire paradigm under which people lived by allowing people without access to information, new access. It changed the power structure and people's view of the world. It allowed knowledge of other new inventions, processes and ideas to be spread. Anything new is only useful if you know about it.
By this same token, I would lax if I didn't say that I think the internet is the printing press of modern times. It has again levelled the playing field in allowing easy access to information. People all over the world can see and hear and read about things they would not have known about before. It has again changed the power structure (small example: I can self publish with the use of the internet, publishing companies no longer are all powerful over what is out there to read) and again opened people's view of the world.