PS/2
If you've ever used a keyboard or a mouse, we'll wager at some point you've dealt with PS/2 connections. The PS/2 itself took over from the delightful DIN socket, which had previously been the port of choice for keyboards. The best thing about PS/2 was its relative compactness.
PS/2 does present a slight problem, namely, how do you plug the keyboard into the keyboard socket and the mouse into the mouse socket? Of course, we hear you shout, "they're colour coded" -- but what use is colour when you're grovelling around underneath a desk at 8:30 at night trying desperately to make the mouse point and the keyboard type?
Sadly, PS/2 was yet another victim of USB, which doesn't care what you plug into it, the electrical slut.
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