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.
The nuclear bomb. Even before we nuke ourselves into extinction, it has had a profound impact on geopolitics. Imagine the huge conventional wars we could have had!