The USB A cable. That cable that you use to connect pretty much anything to some thing else is a simple device, but may belong in the all time ubiquitous technology devices.
While on paper it has been superseceded by superior technology, it still remains in wide spread use. While travelling and in need or a device recharge, the USB port is sought after almost as much as the standard power outlet.
Like the iPod, or Windows XP, the universal serial bus standard proved so successful that even when it should be outdated it continues to thrive.