In addition to being an author, political philosopher, and postmaster, Benjamin Franklin was a prolific tinkerer. He invented swim fins, the Franklin stove, bifocals, the mechanized glass harmonica, and conducted several experiments with electricity which lead to his development of the lightning rod.
Although Franklin did not invent electricity, I think his greatest effect on the world is from his experiments with electric currents. You've probably heard some version of the story of Franklin tying a key to a kite and flying it during a thunderstorm to attract lightning. Franklin conducted this experiment in 1752 to try and prove lightning was a form of electricity and could be conducted. Franklin was very interested in electricity and coined many of the terms we use today, including battery, positive and negative charge, and conductor. In Franklin's time, buildings were often at risk of damage by lightning, but his invention of the lightning rod allowed the electric current of lightning to pass safely into the ground.
Franklin's invention of swim fins changed the way people move through water. Franklin's initial prototypes were round wooden boards which he attached to his feet, but swim fins have more recently been adapted to have a spade shape and be made of lightweight materials like rubber. Any occupation or leisure activity which calls for diving would be much more difficult without the aid of swim fins!
Bifocal glasses have also been a great benefit to the visually impaired—Franklin himself suffered from both near and farsightedness.
The glass harmonica did not experience as much lasting popularity as Franklin's other inventions, and even in its heyday the glass harmonica had mixed reviews. The glass used to make the musical instrument often contained lead, which would slowly poison the person practicing the instrument. Rumors developed that the sound of the glass harmonica caused people to go mad and die, when really it was lead poisoning causing brain damage. Even though lead-free glass can be used today, the instrument has still fallen out of fashion.
One more of Benjamin Franklin's ideas has had quite a lasting impact on the world—Daylight Savings Time. Not every country practices this adjusting of schedules and not everyone is convinced it is a good idea, but it certainly affects agriculture and business around the world.