I'm talking about the air conditioner, whose wide-ranging effects on society are explored at The Atlantic:

Many of the central changes in our society since World War II would not have been possible were air conditioning not keeping our homes and workplaces cool. Florida, Southern California, Texas, Arizona, Georgia, and New Mexico all experienced above-average growth during the latter half of the 20th century -- hard to imagine without air conditioning. In fact, the Sunbelt's share of the nation's populations exploded from 28 percent in 1950 to 40 percent in 2000. And hubs of business and technology in hot regions of the globe, such as Dubai, may never have taken off.

Computers throw off a lot of heat, too. The development of the entire IT industry might not have happened without cooling technologies first pioneered by air conditioning.

The advent of air conditioning has shaped our homes and family life as well. Houses are designed not for ventilation but for central cooling systems. Porches, where they exist, are relics of another age, and few new homes include them. Families gather inside, in the comfort of 72-degree living rooms, to watch TV. Would television have even gained its central place in American family life, were the rooms from which we watch it not so enjoyably cool?