Alan Boyle's Cosmic Log offers a good story on what UFO enthusiasts call the best case of the year, a multiple-witness sighting by employees at Chicago's O'Hare airport. Boyle includes a discussion, with links, about some of the phenomena that can be mistaken for strange-looking craft in the atmosphere.
COMMENT: I've always felt that, in dismissing UFOs wholesale, we're missing some clues about still-unknown or poorly-known atmospheric phenomena. I still think the late Philip Klass may have been on to something four decades ago when he hypothesized a kind of larger, longer-lived cousin of ball lighting. His 1968 book UFOs-Identified maintained that this theory, if correct, could explain most UFOs, given the human mind's tendency to fill in extra details like windows. His later work was much more devoted to the idea UFOs were a mix of psychological phenomena, misidentifications of known phenomena, and hoaxes. In a letter to me about 1991, he said he still thought his plasmas were likely to exist, but were behind only about 1% of UFO sightings.