For me, the best 20th century invention is... computer, of couse. Almost everybody know it.
A little assistant under your desk isn't nothing strange...now. A screen, keyboard, mouse and this little box is known almost by everyone. Why? In the past nobody thought about it as a great device to play, learn and all this stuff. It changed so much and now, with the Internet in every house, everybody know computer and it's strong and weak sides. Some people love it, others hate it but it's true, that it has some advantages and disadvantages.Hovewer it's reaally useful and, most important, easy to use in experiences hands.
For this reason I think about computer as the best 20th century invention and I recommand using it at every step.