Smartphones.

If you told someone in 1980 that just three decades later they will have a portable phone, a video phone, a worldwide electronic teletype, a movie player with a 300dpi screen, a color photo and video camera with a flash, a satellite world map, a music player that holds over a hundred LP albums, dictionaries and translation tools for all Earth languages, train and flight schedules updated in real time, a library of books (one of which an illustrated encyclopaedia that exceeds Britannica in volume and, in some areas, in accuracy), plus thousands of free porn channels and video games, all crammed into a single wireless device that fits in your pocket and costs less than $300 -- would they believe you?

I'm no expert on science fiction, but as far as I can tell, the vast majority of sci-fi authors missed this idea -- perhaps because it's too ridiculous to be believable.

Added: What's even more ridiculous for a layman from 30 years ago is that all this amazing stuff is implemented on the basis of a computer, basically a machine that adds or subtracts binary numbers and stores the results in memory. "Eh? A music player that works by adding binary numbers? Are you okay?"