According to Washington University in St. Louis, the 1904 St. Louis World's Fair saw the introduction of the electrical plug and wall outlet. And as the university points out, you still can't have too many outlets.
Electricity remained a major attraction at the 1904 exposition: The fairgrounds and major buildings were lit with electric lights, and electrical machinery drew large crowds. In the years prior, Westinghouse won bids to illuminate the 1893 (Chicago) and 1901 (Buffalo) World's Fairs using Nikola Tesla's alternating current (AC) system, rival to General Electric/Thomas Edison's direct-current technology. In 1893, Tesla also showcased the first neon lights and phosphorescent lamps (predecessor to fluorescent lighting).
Image credit: Brooklyn Museum/Flickr (World's Columbian Exposition: Grand Basin, Chicago, 1893).