This will sound a little mundane but, FLASHLIGHTS! Particularly bicycle head lights. The prices before LED's were just STUPID. Hundreds of dollars for small amounts of light (which to be fair was the best you could get at the moment). Which were being used for night mountain biking. But all I needed was to get to and from work safely at night, I didnt have $400 for a headlight that would actually let me see the ground in front of me.

BUT, then came the revolution. China started putting out these LED lights that blew everything else out of the water ... FOR CHEAP! In two years light prices went from $400 to $100 for top of the line lighting. US bike light companies were a year or two out before they could re-tool to match the lumens coming out of china. Mind you, the Chinese lights were not always the most reliable. BUT they were 1/4th the cost of a name brand light. So even if it died, you could still buy ANOTHER one for less than the price of a high end name brand light.

And since the LED revolution, things have not changed much. Prices either go down or stay the same and the lumens increase OR the burn time increases. Its just a win win for customers/consumers.