1. The Freemasons

This is the granddaddy of all not-so-secret secret societies. Freemasonry, or "The Craft" as its members call it, most likely has its roots in 17th-century stoneworkers' guilds. Mason lore, however, extends its origins back to biblical times, linking the society to the building of the Temple of Solomon. Freemasonry is split into numerous subgroups and orders, all of which consider God the Grand Geometrician, or Grand Architect of the Universe. At their hearts, these groups are all means of exploring ethical and philosophical issues, and their rituals and symbols are famous (or infamous). Take, for instance, the square-and-compass logo often seen on the backs of Cadillacs. Or the use of secret handshakes, passwords, and greeting postures/gestures called "due guards," all collectively known as the Modes of Recognition. The list of famous Masons is massive, a virtual Who's Who of modern history, explaining the many conspiracy theories regarding the Masons' influence and intentions. Mozart, FDR, Harry S. Truman, George Washington, Mark Twain, Voltaire, Benjamin Franklin, John Wayne, W. C. Fields, and Douglas MacArthur were all Masons. But perhaps the Masons' greatest strides have been made in fast food: KFC's Colonel Sanders and Wendy's founder Dave Thomas knew how to secret-shake with the best of 'em.

2. The Illuminati

Over the centuries, lots of groups have called themselves the Illuminati ("Enlightened Ones"), but the one we're talking about here began as the Bavarian Illuminati. A radical product of the Enlightenment and offshoot of the religion-based Freemasons, the Illuminati espoused secular freethinking and intellectualism and proved a threat to Europe's old order. Although they were officially banned by the Bavarian government in 1784, some claim that they live on to this day in other guises. So, what's the Illuminati's goal? To establish a new world order of capitalism and authoritarianism, of course! They've been accused of manipulating currencies, world stock markets, elections, assassinations, and even of being aliens. One common myth is that the eye-and-pyramid image on the dollar bill is a symbol of the Illuminati watching over us. Nope. It's a symbol of strength and durability (though unfinished, symbolizing growth and change), and the all-seeing eye represents the divine guidance of the American cause. Or so the government says.

3. Opus Dei

This organization has a $42 million, 17-story headquarters building on Lexington Avenue in New York City, claims 85,000 members in 60 countries, and was featured in Dan Brown's bestseller The Da Vinci Code. Now that its existence has been significantly unsecretized, this ultraorthodox Catholic sect has definitely raised its share of eyebrows. Founded in 1928 by Saint JosemarÃ­a EscrivÃ¡ (a Spanish priest who bore an uncanny resemblance to Karl Malden), Opus Dei is the short name for the Prelature for the Holy Cross and the Work of God. The sect (some would say cult) stresses a return to traditional Catholic orthodoxy and behavior, especially celibacy, with members falling into one of three levels. Numeraries live in Opus Dei facilities, devote their time and money to the prelature, attend mass daily, and engage in mortification of the flesh (wearing a spiked chain around the thigh called a cilice, taking cold showers, or flagellating themselves with a knotted rope called "the discipline"). Next come Associates (kind of like Numeraries, but living "off campus"), then Supernumeraries (the rank-and-file members). The group did gain the praise of Pope John Paul II, and has engaged in a lot of charity work. Yet, critics accuse the group of being linked to fascist organizations like Franco's government in Spain, and of anti-Semitism and intolerance, even of other Catholics. [Photo courtesy of DanBrown.com.]

4. Skull and Bones

Top dog among all the collegiate secret societies, Yale's Skull and Bones dates to 1832 and goes by other spooky names like Chapter 322 and the Brotherhood of Death. With a large number of Bonesmen who have attained positions of power, including the president and the head of the CIA, it's no wonder that rumors abound that the society is hell-bent on obtaining power and influencing U.S. foreign policy. The fact that they meet in an imposing templelike building on the Yale campus called (what else?) the Tomb doesn't really help. Bonesmen are selected, or "tapped," during their junior year and can reveal their membership only after they've graduated. But they can never talk about it. The Bones have been accused of all sorts of crazy rituals and conspiracies, including drug smuggling and the assassination of JFK (a hated Hahvahd man, after all). It's even rumored that the skull of Geronimo resides in the Tomb, stolen from its resting place by Prescott Bush, Dubya's granddad. In one of the more commonly known rituals, the initiate spends all night naked in an open coffin, confessing all his sexual experiences to the group. So, who's lucky enough to have made such a confession? George H. W. Bush, George W. Bush, John Kerry, William Howard Taft, McGeorge Bundy, William F. Buckley, and Henry Luce are just a few.

5. The Bohemian Club

This is a weird one. In the majestic forests of Sonoma County north of San Francisco lies the Bohemian Grove, the 2,700-acre wooded retreat of the Bohemian Club, the nation's most exclusive men's club. Every July since 1879, the "Bohos" have gathered at the Grove for a two-week encampment, where they're divided into more than 100 residential camps with names like Owl's Nest, Cave Man, and Lost Angels. Membership has included, well, just about everybody important: Ronald Reagan, Dwight Eisenhower, Richard Nixon (who once called it "faggy"), Gerald Ford, Colin Powell, Dick Cheney, and many CEOs and wealthy business leaders like Malcolm Forbes. Each encampment opens with a robed-and-hooded ceremony called the Cremation of Care, in which an effigy called "Dull Care" (symbolizing worldly concerns) is burned before a 40- foot concrete statue of an owl, symbol of wisdom and the club's mascot. Throughout the week, plays are staged (called High Jinx and Low Jinx), there's lots of eating and drinking (and, reportedly, urinating on trees), and members are treated to speeches called Lakeside Talks. Some opponents go so far as to accuse the group of Satanism, witchcraft, homosexuality, and prostitution, while more reasonable observers object to the Lakeside Talks as national policy discussions to which the public is not privy. But above all, it's seen as a way that some of the elite meet others of the elite, thereby ensuring that they'll all stay elite. All this makes the club's seemingly anticonspiratorial slogan—"Weaving spiders, come not here"—that much more ironic. [Photo of Reagan & Nixon at the Bohemian Club courtesy of Wikipedia.]

6. The Trilateral Commission

While not, on its face, as juicily sinister as some of the other societies on this list, the Trilateral Commission has been accused of all sorts of underhanded shenanigans by its critics. Formed in 1973 by David Rockefeller, the Commission includes over 300 prominent citizens from Europe, Asia, and North America in a forum for discussing the regions' common interests. But conspiracy theorists hold that the Trilateral Commission, along with the Council on Foreign Relations and others, is really just a front for a larger, more sinister order called the Round Table Groups, founded in London over 100 years ago and bent on the creation of a new world order, a global capitalist police state. Yikes! (For the record, some say the Round Table Groups are themselves just fronts for another society, the Illuminati, so who knows?) American members of the Trilateral Commission have included Bill Clinton, Henry Kissinger, Jimmy Carter, Dick Cheney, and Dianne Feinstein.

This article was excerpted from the mental_floss book 'Forbidden Knowledge.'