re: What do you consider the greatest invention?

Anyone who has had a debilitating injury or suffered a chronic or severe illness (or witnessed a loved-one's injury or illness) will say that health is everything. So the answer to this question has to be something that made our lives healthier. So I'll go along with penicillin/antibiotics.

And along the lines of everyone pointing out that electricity was not an invention, penicillin was also a discovery, not an invention.