The Hoofnagles (Mark and Chris) have started a denialism blog Their take on the common denialists found online:
"HIV/AIDS Denialism - seems to feed into some egomania of this particular type of denialist. They frequently make statements about how one day they'll be vindicated, and seen as heroes because they saw the truth first. They also seem to really like inversions, and to feel superior because they believe in something that no one else does. Other conspiracy theorists, such as 9/11 conspiracy theorists, I think are similar. There is an egotistical appeal to possessing "secret" knowledge or holding controversial opinions. Basically, I'm calling them assholes.
Global Warming denialism - motivations seem to range from financial (industry, their lobbyists and think tanks), to individual cognitive dissonance. Many global warming denialists that argue from a non-financial standpoint seem to fear the changes that reducing a carbon footprint entails, and are concerned about losing quality of life. Others, I think, suffer from the same egomania as the HIV/AIDS denialists like Monckton. Still others, most recently Falwell, seem upset from a religious perspective as it suggests humans could somehow harm God's creation or worse, that global warming might be a positive sign of Armageddon.
Creationism/Intelligent Design Denialism - Almost exclusively religious objections, stemming from cognitive dissonance from fear of non-literal interpretation of the bible. The insistence on believing the truthfulness of some of the more absurd stories of the Bible such as Noah's Ark seems indicative of a certain stubbornness and fear of upsetting the fragile balance of the literalist's world view. The idea that the Bible might contain metaphor, rather than absolutes, is therefore terrifying and ideas such as evolution, that appear to negate the creation myth must be opposed at all costs.
Holocaust Denial - Hopefully we won't have to cover this disgusting kind of denialism as much. Its motivations are perhaps the clearest of them all. It's just plain Antisemitism.
Anti-Vaccination denialists - I have less experience with these but it seems mostly to be fearfulness of science, and a propensity towards believing in what Orac would call "woo". A small amount also seems to be paranoia or paranoid personality disorder. Finally, I think many of the parents of autistic children seem fearful of genetic or environmental causes of autism that might implicate their culpability in their child's illness. Sadly, throughout the history of autism parents have often been blamed, specifically absent fathers and cold or "Frigidaire" mothers were implicated. So it's understandable if people still feel some stigma or guilt from such a diagnosis given such a cruddy history from the psychologists on that one. Many parents would like to believe in something, anything that explains why their child has been singled out by nature to have autism. Having something to blame, like a vaccination, therefore becomes emotionally very appealing and alleviates some of the helplessness or misplaced guilt they may feel.
Animal testing denialists - I am somewhat uncertain about inclusion of some animal rights activists (ARAs) into the denialism camp. My reasons for doing so include the general dismissiveness I've seen of science by ARAs. For instance, saying things like science can be done without animals (or worse on a computer), claims from PETA that chickens are as smart as dogs or babies, that dogs are actually vegetarians (oy), and any number of discussions in which they imply that animals or animal models say nothing helpful about biology. Their motivations, for the most part, are more noble, they're interested in alleviating suffering, all suffering. That this is impossible, misguided or unwise is not important, and if they have to lie about science then so be it."