Too many to mention. Personal opinion may rank some, as more useless than others. They soon get forgotten, as no one will adopt or use them.
Two spring to mind.
An ashtry on a motorcycle
and a chocolate teapot.
There is a few inventions that are not very useful. The most known invention that is not useful is the toilet paper roller.
It will have to be Magikarp.
The wheel.
This is the most useless site that ever exsisted
that i just had a cheese sandwich, and i have no head.
the wheel.
Iphone 5
It is like the most educational invention EVER!
The wheel
musical instruments...where would we be without music?
WikiAnswers is definitely NOT the most useless website. WikiAnswers is very useful, and can only be very useful through the work of your volunteers (users). WikiAnswers is useful mainly for those generic, mind boggling questions, and not the complex, detail oriented questions. Keeping questions as simple as possible will make WikiAnswers the most useful site ever. WikiAnswers is only useless through the eyes of those who are not patient and do not get immediate answers, and/or their question is not on the site. because you don't ask the IMPORTANT questions Wikianswers is not useless, it's just the contributers that are useless.
Today's electricity systems are based on Tesla's invention no matter where you go. The induction motors is a part of most machineries in most work places. Everywhere you look, there is a Tesla invention. When you start your car, there is a Tesla invention known as the electric starter. The Tesla impact will be there for ever.