UFO researchers have actually contributed in an indirect way to some useful scientific speculation - their cataloguing of lights in the sky has led some geologists to believe that tectonic forces might be generating some of these 'earth lights' ( . . . or UFO's), and ball lightning is a common electrical phenomenon that has been added to the list of possible causes for UFO sightings.

My favourite theory is some are unknown species of animals living in the atmosphere which are visible under certain circumstances, have a look here and here. The best is the space worm filmed by the bald astronaut whose name escapes me.