The Church of Scientology is possibly the largest and most well-known "dangerous religious cult" in the United States, but is is actually not the biggest "lunatic fringe" cult in the world. That honor may go to the Unification Church, which is based in Korea and has been quietly working to gain political control and power in the United States since at least the 1960s.
The Unification Church is known to most people as the "Moonies" for good reason: anything that disagrees with the opinion of its leader, Sun Myung Moon, is quickly ejected. These guys love to talk about faith and the Divine Principle as it was received in a revelation by Moon, though they're really hush-hush about Moon's announcement in 1990 that he is the Messiah. The Moonies aren't mentioned very often in mainstream media these days…and there's a reason for it. It's not because they've reformed their ways (they haven't), but rather because the Unification Church has been working on a long-range goal of infiltrating American media and politics. To this extent, they've had a notable degree of success. After buying and pouring millions of dollars into the Washington Times newspaper to make it a "respectable" journal (they also publish Insight magazine, the publication that funded Paula Jones' "sexual harassment" lawsuit against President Clinton), the Unification church bought the UPI news network in the year 2000…thus gaining even further subtle influence in mainstream politics.
In April of 2008 at the age of 88, Reverend sun Myung Moon officially passed on the leadership of the Unification Church to his 29-year-old son, Reverend Hyung Jin Moon. As of this writing, there has been no word if he will follow in his father's footsteps and declare himself the Messiah once Dad kicks the bucket.
Steve Hassan maintains a lengthy list of front groups for the Moon organization, while former members of the Unification Church make their stories available online at the X-Moonies Web site.