The Illuminati were founded by Dr. Adam Weishaupt, a professor of Canon Law. A former Jesuit, on May 1, 1776, he founded a secret organization within the Freemasons, another secret organization. So essentially, the Illuminati is a secret within a secret.

In 1785, the Illuminati was suppresed by the Bavarian Government because they caught wind of a plot to overthrow all the Kings of Europe and to assassinate the Pope.

The Illuminati was also managing the Napoleonic Wars. After overthrowing the French Monarchy, the Illuimati placed Napoleon in power. After the Battle of Waterloo, the Illuminati quickly spread rumours in England that French had won. (Back then, due to a lack of technology, communication was limited.) The result? The Stock Market crashed and the Illuminati, whose members included George Washington, Benjamin Frnaklin, and Thomas Jefferson, bought all they could.

The Illuminati is also found in the Media. In Laura Croft: Tomb Raider, a group of villains who want to take over the world call themselves the "Illuminati." The Holy Book of Discordianism, label the Illuminati as a group of people trying to take over the world.
In Resident Evil 4, a group called Los Illuminados creates parisites to rule the world. The Story line of Street Fighter III revolves around an organization called the Illuminati trying to make a, quote, "New Utopia." And the list goes on and on.