The Tunguska Explosion took place on June 30th 1908, exactly 100 years ago today. Although it is almost certain that it was the result of an small asteroid or comet exploding in the atmosphere with an estimated yield(*) equivalent to 20 million tons of TNT (i.e. over a thousand times more energy than released by the atomic bomb dropped on Hiroshima), many mysteries still remain.
Scientific American had an interesting article on Tunguska in its June 2008 edition, on the search for a crater. Space.com has another article worth a look, which has some fun at the end by mentioning some of the more outlandish hypotheses put forward over the years: an alien space ship blowing up, a mini-black hole passing through the Earth, or Tesla's experimental death ray.
(*) The explosion of 1 ton of TNT is equivalent to the release of 4.18e9 Joules of energy (see here), so roughly the Tunguska explosion was about 8.4e15 Joules, or 8.4e22 ergs. By way of comparison the luminosity of the Sun is 3.845e33 ergs per second.
[The image is of trees flattened by the explosion, taken twenty years afterwards by the Kulik expedition. Source: APOD for 2007 November 14]