Self driving cars. The amount of time saved for humanity will be absolutely staggering and free up people to spend more time accomplishing more things.
Agreed that a lot of time will be freed up, but I don't think the majority of people will use this time productively. Most people will just end up watching Netflix in their cars instead of focusing on the road.
The real revolution for self driving cars will be what it does to deaths in the 18-35 demographic. A quick Google search turns up this (https://www.verywell.com/top-causes-of-death-for-ages-15-24-...) article that confirms that for the 15-24 demographic automobile accidents are the leading cause of death. Think of how many productive person-years we'll gain if self driving cars are even just 10% safer than driving yourself (and by all measures I've seen, they're way safer than that)!
Edit: To understand the real impact, I've always enjoyed this hypothetical future conversation...
Future Kid: You mean to say that people used to operate machinery weighing several tons? Traveling at high speeds? With a minimum of training?
I think this is the biggest part, and less the "sleep in your car while you commute for 5 hours each way!" which sounds fucking terrible.
Imagine the amount of space and time saved by having 1 enormous lot on the outskirt of town, like an airport. You walk around your dense urban area, call your car, 10 minutes it's there, it drops you off, and parks itself. No circling the block, no terrible big box concrete hellscapes, just Park Slope without the frustration.
I highly recommend this Benedict Evans post on the second-order consequences of electric & autonomous vehicles: https://www.ben-evans.com/benedictevans/2017/3/20/cars-and-s...