Light
Obviously, Nikola Tesla was not first to invent the light bulb, but he did find new ways for light to be harnessed and distributed. He created fluorescent bulbs in his labs roughly 40 years before the industry invented and sold them.
The first use of neon signs was at the World's fair, when Tesla bent glass tubes into the names of famous scientists. More impressive, however, is the Tesla Coil, which the big industry would tried to suppress. The Tesla Coil was the concept that earth itself is a magnet that generates electricity (which is known as electromagnetism) that utilizes frequencies as a transmitter. The only thing that is needed to use it is a receiver, which would be similar to a radio.