Really surprised no one's mentioned the Internet - maybe you're just blaming this invention on Al Gore! Really, though, I would go with chem/biological weapons. I also don't tend to jump on board the atomic bomb hate; you gotta remember it also led to the development of peaceful nuclear power and other modern science. Also, it's hard to argue against just regular firearms- sure, some automatic machine gun might have no purpose other than to kill, but historically from muskets to modern times guns have had many purposes. But then, chem/bio weapons really seem to have absolutely no other purpose or good in them. As far as inane gadgets/random meme-like inventions go there are too many to name; one thing that famously has gotten a lot of hate from the general public is the alarm clock.
