http://www.straightdope.com/columns/030228.html
Another theory advanced that is only slightly less silly than UFO theories:
It was a mistake of Tesla’s High Energy experiments at Wardenclyffe Tower on Long Island.
I’ve seen this way to many places. Stories like this obscure his real achievements.
Tesla was an enigmatic genius, but some conjecture has really gotten out of hand
(From Wikipedia)
The Wardenclyffe Tower
It has also been suggested that the Tunguska explosion was the result of an experiment by Nikola Tesla at his Wardenclyffe Tower, performed during Robert Peary’s second North Pole expedition. Tesla had claimed that the tower could be used to transmit electromagnetic energy across large distances. Allegedly, he had sent a communication to Peary, advising him to be on the alert for unusual auroral phenomena encountered as he attempted to reach the North Pole.
However, the workings behind Tesla’s Wardenclyffe Tower are not well understood, and it is not certain if Tesla ever used the facility for this purpose. It is unknown if the facility could produce energy and transmit it longitudinally to produce the Tunguska event, an equivalent to a thermonuclear explosion; the atomic nucleus was not even discovered until the following decade (though in 1891, on the struture of the aether and electromagnetics, Tesla stated that there was "an infinitesimal world, with molecules and thier atoms spinning and moving in orbits, in much the same manner as celestial bodies carrying with them static charges).
Regardless if it was possible for the facility to produce such an effect, the main contention that Tesla was not responsible for the Tunguska event is the it occured at about 7:17 AM. Given accounts (if they can be trusted) of when Tesla performed his experiments in the evening of June 30th. Tesla’s facility was 1/4 a day ahead of Tunguska time. Tesla’s Wardenclyffe experiment would then be the day after the Tunguska event (which occured around 1:00 in the morning on June 30th [New York time]).