The Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) home page, publishers of The Skeptical Inquirer.
The Straight Dope: "Fighting Ignorance Since 1973 (It's taking longer than we thought.)" A favorite column of mine from the Chicago Reader, now in Cyberspace.
The Earth's atmosphere is large and complicated. It is therefore not surprising that occasionally phenomena are observed in it that are not immediately understood. However, just because something unusual is seen in the sky does not imply that it is an extraterrestrial spaceship. Some references, both hard copy and on-line, follow.
Philip J. Klass, UFO's Explained (New York: Vintage Books), 1976. Klass was a senior editor of Aviation Week and Space Technology. An on-line archive of his Skeptics UFO Newsletter is available at The Klass Files.
James Oberg, UFO's & Outer Space Mysteries: A sympathetic skeptic's report (Norfolk: Donning, 1982). Oberg has written extensively on space travel. He is particularly well known as an expert on the Soviet and Russian space programs.
Robert Sheaffer, The UFO Verdict: Examining the Evidence (Buffalo: Prometheus Books, 1981).
The UFO Skeptic's Page is where Schaeffer's analysis of UFO stories can be found on-line.
A belief that SETI (the Search for ExtraTerrestrial Intelligence) is worthwhile does not imply acceptance of UFO's as extraterrestrials, see why the SETI Institute does no UFO Research.