He developed the alternating current system, making it possible to transmit electricity over vast distances, and worked on wireless communication and energy transfer. He was a brilliant, but also very eccentric thinker, claiming to get visions and displaying odd behaviour in public like an obsession with personal hygiene and pigeons. Maybe the more enigmatic parts of his personality make him such an interesting subject for conspiracy theories. Tesla is credited to have worked on unknown energy-sources, caused the Tunguska explosion with his "death-ray" prototype, and supposedly worked on an earthquake-generator.
In 1896 Tesla was working on oscillations for wireless energy transfer. The idea was to build a steam-powered oscillator, able to create various changing frequencies. If the frequency matched the resonance frequency of a receiving device, this device should transform the mechanical oscillations back into an electric current.