In 1952 and 1957, Martin Gardner wrote Fads and Fallacies in the Name of Science, his debunking classic. It's interesting to see wh

Flat and Hollow - Not many flat-earthers in the more industrialized areas, though there are some Islamist flat-earthers.
Iraqi TV Debate: Is the Earth Flat? - YouTube
The Flat Earth and Other Crazy Ideas of Modern Islamists

A somewhat less retrograde cosmological idea is geocentrism, but it has some recent advocates:
Geocentricity
Galileo Was Wrong - a Catholic fundie's site

Monsters of Doom - Immanuel Velikovsky returned to public view in the late 1960's and early 1970's when he and his followers claimed that spacecraft discoveries have vindicated his claims. The American Association for the Advancement of Science even had a conference on his theories, with IV, Carl Sagan, and other notables giving talks. When preparing his talk, CS discussed IV's theories with a distinguished professor of Semitic literatures. The professor stated that what IV stated in reference to those literatures was nonsense, but he was impressed with all the astronomy. CS had the opposite impression. But since then, it's dropped out of sight.

Despite being big in Germany in the early 20th century, Hanns Hoerbiger's Welteislehre (Cosmic Ice Theory) has no supporters with any Internet presence that I've been able to discover.

In mainstream geology and planetary science, however, catastrophism has been successfully revived. This was not a result of armchair debates, but of improved techniques for identifying various catastrophes, like asteroid impacts. In fact, the favorite hypothesis for the origin of the Moon is nowadays a collision of a Mars-sized object with the early Earth.

The Forteans - Not much recently.

Flying Saucers - Nowadays usually called UFO's - Unidentified Flying Objects. It turned up that the US Air Force was investigating sightings out of concern that some of them may be secret Russian airplanes. The USAF stopped in the late 1960's after concluding that they were not a threat to the US's national security. Most recently, the proliferation of digital cameras and camera-equipped smartphones has apparently led to a drop-off in UFO interest.

MG briefly mentioned contactee George Adamski in an update, noting that another UFOlogist, Donald Keyhoe "does not, like Adamski, claim to have ridden in a saucer where he conversed with a voluptuous golden-sandaled Venusian." GA and some other contactees reported that the UFOnauts were very benevolent and very concerned about us and our destructive ways. Not surprisingly, mysterious-object UFOlogists have looked down on the contactees as the lunatic fringe of their movement.

UFO abductions came later, starting with Betty and Barney Hill in 1961. The UFOnauts liked to experiment on the people that they abducted.

Zig-Zag-and-Swirl - Lawsonomy Home Page - not a very big site.

Down with Einstein! - As MG notes, a lot of physics crackpottery is very dull. But George Francis Gillette has had successors like Archimedes Plutonium and the Time Cube guy.

Some 19th cy. physics crackpots had advocated particle theories of sound, and they got a curious sort of vindication from quantum mechanics. Sound is also subject to wave-particle duality, and the particles of sound have even been named "phonons". However, sound also has wave properties, contrary to what those crackpots had believed.

MG had noted a progression from being anti-Newton to being anti-Einstein while often claiming to restore Newton. Most recently, a certain Farsight has claimed to be restoring both Newton and Einstein against more recent theories.

Sir Isaac Babson - His Gravity Research Foundation: Home is still in operation, though nowadays financing work on mainstream theorizing about gravity.

Dowsing Rods and Doodlebugs - Still with us, like BBC News - Newsnight - Export ban for useless 'bomb detector' that was sold to Iraqi military and police forces.

Geology versus Genesis - Sad to say, creationists are still with us in a big way. Some of them have even tried to camouflage their creationism by renaming it "Intelligent Design".

Lysenkoism - By the 1960's, it was gone from the Soviet Union.

At marxists.org is T. D. Lysenko Reference Archive It has his 1948 speech Soviet Biology, where he ridiculed the notion of a hereditary substance. But at that time, the Mendelist Weismannist Morganist idealist biologists were closing in on its nature.

Apologists for Hate - Race pseudoscience is still with us. In the late 1960's, William Shockley showed that as a psychogeneticist, he was a great transistor designer. He and some others had claimed that inheritance of IQ ability and black people having lower IQ scores meant that black people are naturally dumber than white people. Also, white racists have been joined by black racists like Leonard Jeffries with his theories of good "Sun People" and bad "Ice People" (guess who).

Atlantis and Lemuria - Ignatius Donnelly's mid-Atlantic Atlantis evidence was the Mid-Atlantic Ridge. It turned out to be a place where new oceanic crust is being formed by underwater volcanism. Its crust is all oceanic crust, like the crust where James Churchward's Mu had allegedly been.

The Great Pyramid - end-of-the-world speculators have recently turned to pyramids in Central America.

Then Medical Cults and several other chapters, with oodles of quackery and altmed therapies from back then.

Orgonomy is about Wilhelm Reich's orgone therapy. Though it's dropped out of sight, some people still practice it.

Dianetics turned into Scientology, and it's had a much more successful history.

Then some other oddities and ESP and PK. That's also had a distinguished career, with Uri Geller notably bending spoons. But that also seems to have faded. Princeton Engineering Anomalies Research closed in 2007.

Finally, Bridey Murphy was an early case of past-life regression, something done by certain therapists and New Agers.

Excellent round up! Us Forteans are still going strong though. Fads & Fallacies is a superb book, and Gardner's maths books are fun as well

Hanns Hoerbiger's Welteislehre (Cosmic Ice Theory)

Do we need equivalents of Landover Baptist for these ideas? Should not someone be collating them and conserving them and accidentally letting them loose in the wild?

Imagine the chaos we could have caused in five hundred years!

Clive Durdle wrote:

Hanns Hoerbiger's Welteislehre (Cosmic Ice Theory)

Do we need equivalents of Landover Baptist for these ideas? Should not someone be collating them and conserving them and accidentally letting them loose in the wild?

Like the George Jammal Noah's Ark hoax? Sun Goes Down in Flames: The Jammal Ark Hoax He treated some wood with various sauces and he then passed it off as wood for Noah's Ark.

I should note that advocates of the Welteislehre can claim vindication from the abundance of ice in the outer Solar System. Lots and lots and lots of ice. But it's no credit to their methods. Alpenfestung - PSEUDOSCIENCE IN NAZILAND by Willy Ley -- has some stuff on the WEL.

One of his pupils, an architect, told awed and large audiences that Hörbiger's information about the true state of the Universe was based entirely on intuition. As a boy, he said, the Master had a small tele-scope with which he looked at the Moon. Then, suddenly, he realized that what he saw was ice, cold ice, the whole Moon was made of ice. He glanced at blinding Venus which was still in the sky; Venus too was cold and brilliant ice. Years later. Hörbiger was asleep, dreaming about astronomy. He saw the Earth as a pendulum suspended from a luminous thread and saw it swing, in longer and longer swings. It swung to Jupiter, and to Saturn and be-yond, but when it swung to three times the distance of Neptune the string broke. Hörbiger awakened and realized that the Sun's attraction stops at that distance.

When someone criticized him about how this or that assertion of his did not work out mathematically, he'd respond with the likes of "Instead of trusting me you trust equations! How long will you need to learn that mathematics is valueless and deceptive?"

Pseudoscience in Naziland discussed some other fields of crackpottery.

One of them was Pendelforschung ("pendulum research"), and it was a form of dowsing. One hangs some small object by a thread that one holds between one's fingers. It got used for lots of things, like this:

Other groups (of the German navy) including officers of flag rank, supported Pendelforschung: a large map of the At­lantic was spread out horizontally, with a one-inch toy battleship as test object. A pendulum, consisting of a cube of metal about one cubic centimeter and a short string, was swung above the battleship. If the pendulum reacted, it proved the presence of a true battleship at that location.

Another was the Hohlweltlehre, the Hollow Earth Theory. Like Cyrus Reed Teed's Koreshan Universology, it features an inside-out Universe, where the Earth surrounds everything else.