Nuclear weapons, or any implement of war. Imagine the financial outlay and intellectual resources mis-applied over the centuries. Of course, this would mean we either ignore or show superhuman restraint in using various technologies for other, more beneficial purposes. It seems unlikely that this would be possible, given current human tendencies.
The first thing that came to mind was cell phones, but that wouldn’t make sense. Beyond the distraction they cause among our youth, they’re also quite useful, some might say indispensable.
How about pharmaceutical commercials?
3 Cents' 