According to Urban Arcana a number of organizations/companies have been started in the "real world" by people from D&D worlds who got stranded on Earth by the low-magic levels here. UA posits that due to planar and magical conditions over the past few centuries, it's much easier for beings from D&D worlds to come to Earth (via the Plane of Shadow) than it is to go to a D&D world from Earth.
Chapter 6 is basically a huge list of organizations, but the ones that had a direct tie to D&D worlds were these:
A fairly new (as of 2003) fast food chain called The Prancing Pony which has a medieval fantasy theme with restaurants dressed up to look like taverns. Several elves, dwarves ect. stuck on Earth work there as "cover jobs" where they don't have to try to hard to hide their true nature.
Epsilon Alpha is an elite fraternity at some Ivy League schools, in reality it is a cover for a cult of Wee Jas known as The Beloved.
The Church of Pelor is a new-age Pagan religion started a few decades ago by stranded followers of the Sun deity from Greyhawk as a way to keep the faith. No central hierarchy, just local congregations composed mostly of curious mundane folk, with a few people from Greyhawk showing up when they recognize the name and holy symbol. Lots of emphasis on community service, does a lot of charity work.
Cirque des Moitie is a thinly veiled substitute for Cirque de Solei. It was founded by a party of stranded halflings and a few dwarves who wanted to find legitimate use for their skills on Earth (apparently being a party of mostly bards and rogues), with a focus on their tumbling and performance skills.
The Corsone Syndicate is a crime family secretly run by an illithid who was fleeing his own kind (apparently a criminal even among illithid), and fled to Earth, arriving "five years ago". The book was published in 2003, so he probably arrived on Earth in 1998.
The Displaced is publicly seen as a large fantasy & sci-fi fandom group who pretend they are "trolls and elves" while doing community service (and kind of get looked down on by mainstream charities), but is actually a mutual-aid society of displaced persons from other planes stuck on Earth. Kind of a fantasy stand-in for the 501st Legion.
Draco Industries is a megacorporation run by an Efreeti who has been on Earth since antiquity. He craves power, and wants to rule Earth as a Sultan.
The Fraternal Order of Vigilance is an organization with lodges in many cities and towns (like many other fraternal lodges like the Masons, Knights of Columbus, Elks, ect.) Their senior leadership is quite well aware of the existence of the supernatural and beings from other worlds, and encourage their members to be vigilant against crime and immorality. A small number of members are inducted into the inner circle where have the presence of the supernatural revealed to them. The biggest problem with the F.O.V. is they perceive all extraplanar beings as a threat (presumably having dealt with too many demons, illithid, medusae, ect) and being an elf or dwarf is just as bad as being a demon. Definitely not above framing somebody for a heinous crime just because their ears are pointed.
Heirs of Kyuss is a loosely organized cult of necromancers. They seldom have meetings, communicating mostly online and through coded personal ads.
Infinite Serpents is another monstrous cult, relatively new, apparently lead by a Yuan-Ti
The International Guild of Laborers is a massive overarching labor union (ala the Teamsters or UAW) that is lead largely by extraplanar visitors and magical beings and secretly makes an effort to try to extend union protections to workplaces with supernatural beings.
The Knightly Order of St. Bartholomew is a religious order dating to the middle ages. It is a quiet arm of the Church that exists to study various things that would be called heretical and blasphemous. In reality they are mundane human scholars who have the largest library of arcane lore and the best understanding of the nature of the supernatural of any Earth natives. They believe these are "things man was not meant to know", and are not spellcasters themselves though.
Knights of the Silver Dragon was founded in the dark ages in Ireland as a religious order of Knighthood, which became a more militant order based in France by the time of the Crusades, eventually becoming a fraternal brotherhood by the time of the Enlightenment and playing a part in the American Revolution. Founded when a silver dragon named Wynnefalshond arrived on Earth. Taking human form and identity, he realized that the people of Earth had little protection from the more powerful monsters that would sometimes arrive on Earth, so he began to secretly train and prepare mankind to defend themselves from horrors from beyond, while keeping the masses of humanity from panicking if they knew what was really out there. Secretly, the entire Order is Wynn's long-term plan to find a way to go home, hoping that the Knights will one day find a portal or way to send him back.
Markova Enterprises is a media conglomerate run by a Drow who arrived on Earth in the early 20th Century. Not a criminal organization, they go out of their way to stay quite legitimate, but they are definitely trashy and questionable in terms of their choice of programming.
Paranormal Science and Investigation Agency is a small agency of the U.S. Government founded in the 1950's, buried deep in line-items in budgets and odd corners of organizational charts. They are the U.S. Government's main line of defense against magical and psionic threats.
St. Cuthbert's House is a religious charity that operates quite openly in major cities. They provide free medical care, food, and homeless shelters. It was NOT founded by a Cleric of Paladin of St. Cuthbert, instead it was founded by a street youth named Donny Sinclair who was mugged and left for dead on the street. . .when a stranded Cleric of St. Cuthbert found him and healed his wounds, telling him that St. Cuthbert had found him worthy. This religious experience changed the young man, who devoted his life to charity and created the St. Cuthberts House organization (eventually becoming a divine spellcaster of St. Cuthbert himself, his listed stats make him a 5th level Acolyte).
Sons of Tyranny is an outlaw biker gang founded several decades ago that is also a cult to Hextor (apparently the founders of the gang/cult were ordained by an aspect or avatar of Hextor himself who went to the great effort to appear on Earth).
The Swiss Juncture of Gnomes is a small cabal of Gnomes based out of Zurich that acts as a consulting firm on technology and science.
