There are many speculations, almost as old as the Bible text itself, regarding the meaning of 666 or to whom it is referencing. The Apostle John wrote Revelation around 95 A.D. The Catholic Church states that Irenaeus (130 - 202 A.D.), an early church "father," wrote about whom this number might reference. He stated that the Greek word "Lateinos," when given their corresponding Greek values and added up (30 + 1 + 330 + 5 + 10 + 50 + 70 + 200), equals 666. The word itself means "Latin man."
Bullinger, in his monumental book entitled "Number in Scripture," states that the ancient Romans used only six letters of their alphabet in their numeric system. The letters they used were I, V, X, L, C and D (the letter M, according to Wikipedia, was not assigned a value in Roman times). If we add up the value assigned to all these letters (I =1, V = 5, X = 10, L = 50, C = 100 and D=500) we arrive at the number 666. Another theory, discussed by the IBSE, states that many scholars believed that 666 referenced Nero Caesar as his name, written in Hebrew, adds to this total.
Here is something to consider. God is the Being who assigns a human number, 666, as an identifier on the Beast. Biblically, the number 6 represents man's incompleteness or imperfection, as it is one less than seven, which represents God's wholeness and perfection. So, why wasn't a single 6 used to denote the Beast? It could be that three 6's are used to demark something that is far more than just humanly imperfect. They are used to signal something that is exponentially evil.
The one-world system set up in the End Time will be both terrifying and breathtaking. For true Christians, meaning those who have faith, obey God and keep his commandments (Revelation 14:11 - 12), it will be a time of martyrdom. For the rest of humanity (at least for a short time) the world will experience its greatest prosperity (read Revelation 18:9 - 19).
Make no mistake, however, the Beast will be the unchallenged head (Revelation 13:4) of a world-ruling system meant to exalt and worship him. Obedience to him will be enforced on an unprecedented scale. It is such a system, ruled with the attitude of a ravenous wild animal, which God seeks to warn us about through 666 in the Bible.
Trivia related to the meaning of 666
Highway 666 in the United States was renumbered to Highway 491. This was done because it was suggested that the high rate of accidents on the street was due to Satan. Others conjectured, however, that the high accident rate was likely due to people trying to steal the highway signs and sell them online at places like EBay.
An American city in the state of Louisiana petitioned, and had changed, their phone area code from 666 to 749. Three sixes, however, are considered lucky in China, where they are commonly displayed in shop windows. The pronunciation of the number 6, in Chinese, sounds like the word used for "easy" and "smooth." It is usually repeated three times for maximum effect.