Exclude all web/mobile/application/computer software interfaces.
closed as not a real question by ChrisF, Vitaly Mijiritsky, Charles Boyung, Patrick McElhaney Mar 31 '11 at 13:13
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It's hard to name these things because, as with all well-designed items, we don't think of them as well-designed, because they so naturally fit into our lives. Think about toothbrushes, doorknobs, drawers, light switches (though they could be better, yes). They all have user interfaces (because they can be interacted with by a user) and they all are nearly perfect.
Also, look at musical instruments. They evolved for hundreds of years and have achieved a high level of perfection.
I'd say either:
An ATM machine - although they are a bit more complex nowadays, but back when they only offered withdrawal of cash (not phone topups, deposits etc that they offer now).
Pedestrian traffic crossing signals - single button interface with clear feedback to the user as to what to do.
It could be argued that neither of these require training to use and while not 100% instinctive are pretty darn close.
I like wall-mounted manual PET bottle compactors. The sound of plastic crounching with just a minimal force application while knowing that you did something good for the environment is really satisfactory.