The greatest thing since...

[Jenise]

In July 1928, when the Chillicothe Baking Company in Chillicothe Missouri put its new bread slicer to use, the idea of packaged pre-sliced bread was revolutionary. The slicer was invented by a friend of the bakery's owner, a local jeweler and notorious tinkerer named Otto Rohwedder. He also came up with ways to minimize sloppy loaves and preserve freshness once a loaf had been sliced--a bread wrapping machine that immediately sealed sliced loaves in a bag.

Americans were quick to adapt. By 1930, just two years later, pre-sliced loaves could be found in every U.S. town thanks in part to Wonder Bread, which came to dominate the sliced bread market. By the 50's "best thing since sliced bread" came into the vernacular to describe new inventions like toasters and television sets. (From an article in Food & Wine)

I have also heard, "best thing since canned beer." A quick look at the history: The development of canned beer started in the early 20th C but was delayed by Prohibition. It eventually happened in the mid-1930s. The cans were popular -- weighed less than glass, didn't break, got cold faster -- but you needed a church key to open it. That led to cone-top cans, which had a crown cap and, incidentally, were easier for small makers to produce because they could adapt their bottling lines more easily.

But it was WWII that really sold the idea of canned beer (to thirsty G.I.s).