Flash drives for sure. I remember being in a computer class back in, oh, 2001? I bought my first flash drive from Radio Shack and remember getting all excited about having 32 MEGS OF STORAGE in a drive several times smaller than a floppy. It lasted longer than a floppy disc, too, until the USB port started to separate from the flash board from too many pluggings and unpluggings. Great technology, and it only got better and better. I think I paid, oh, forty dollars for that flash drive with 32 megs of storage. Now you can get them with 32 GIGS for a fraction of that price.
USB is terrific because it's so versatile. Keyboards, flash drives, mouses, joysticks, printers, scanners... the USB can handle all these peripherals, and so many more. Sometimes it recognizes what you've plugged in right away, without the need for a driver! The USB standard is twenty-three years old at this point, and many of the legacy devices from that era which use USB can still be used today. It's the one technology that's both future proof and past proof.
If we're going to go back a ways, voice synthesis is something we take for granted now but seemed utterly A-M-A-Z-I-N-G in the early 1980s, when many computers and video game systems were first using it. You had to have a peripheral, but the voice was crisp enough to understand, and it did legitimately contribute to the experience. You'd get important audio cues from the Gomer Pyle-ish voice in B-17 Bomber, and KC Munchkin's friendly announcer voice adds personality to KC's Krazy Chase. It's just not the same without hearing KC belt out "HA HA HA! InCREDible!" at the end of every round.
I remember after I bought my NES in 1988, that I went into the store, looking for new games to add to my collection. I saw Ikari Warriors II: Victory Road behind the glass, complete with a starburst proclaiming "featuring voice!" on the box. I wasn't sure if you had to buy a voice synthesis peripheral for the NES to get that voice, and the clerk at Meijer didn't know either. Turns out it was build into the cartridge with no extra hardware needed, which I suppose spelled the beginning of the end for the voice synthesis boxes of the 1980s. I suppose it's good that we no longer need them, but their slightly robotic speech did add a quirky zeerusty charm to the early video game experience.