If you ask me? Mobile/WiFi internet… The way and amount of time we use our phones had changed A LOT since their diffusion. I guess the release of the iPhone changed our idea of what a phone is too
Edit: when I say modern world I’m referring to the last 50 years. So stuff like “the electricity” or “the telephone” doesn’t count.
Cars shaped city planning, housing and by consequence our lifestyle, making us more dependent on them to get through your day. You can see it expecially in European cities that were built in medieval/reinassance times: if you live and work in the older parts you can totally do without a car. If you move a few kilometers out, not having one becomes a real handicap
You can see this in the U.S. as well.
In many parts of the world, though, I wouldn’t say cars per se, but definitely public transportation. A lot of people can’t afford cars in the world, and they still benefit from the invention of the internal combustion engine.