In 2000 I held a competition on my community radio breakfast program to find the seven wonders of the modern world. One guy suggested "Sliced Bread" because if we didn't have it we wouldn't be able to say "the best thing since sliced bread".
But in the spirit of your question, I'll opt for "the best thing since transistors". Why? Because my first portable radio weighed 20 lbs of which 8 lbs were batteries. Then transistors replaced valves and we could all carry our music so much more easily.
