What Thomas Edison invention do English speakers use every day, according to John Lloyd and John Mitchinson, authors of The Book of General Ignorance? The word hello.
That may be a slight exaggeration, according to other sources I've consulted, but only slight.
You see, Thomas Edison wanted a word to use as a greeting while he was testing Alexander Graham Bell's telephone prototype. While Bell preferred the far more whimsical ahoy hoy, Edison felt that hello was easier to understand from a distance.
While The Book of General Ignorance hasn't convinced me that Edison coined the term, he does seem to be responsible for hello being the standard telephone greeting. Before that, telephone operators greeted callers with Who are you?, which seems an awfully rude way to greet someone.
Eventually, after hello became the most accepted form of telephone greeting, telephone operators came to be known as hello girls. According to General Ignorance, the first recorded use of name tags bearing the phrase Hello my name is... was at an 1880 telephone operators' convention.
This is not to say that hello was an entirely new invention. Variations on hello have been around a long time, according to the Oxford English Dictionary. There was hallo, originally used on ships to urge folk along, funny because it comes from the word holla, which was a command to stop. This led to halloo, a word meaning to shout. Hallo, in turn, gave rise to holler. This all came full circle in the early 2000s with the slang word holla, a form of greeting based on the word holler.