numerologically speaking, 666 has often been expressed to be "the number of the beast". as huey said in the premier episode of the boondocks (which i really enjoyed): "look, I'm trying to tell you that ronald reagan was the devil!" he insists. "ronald wilson reagan? each of his names has six letters! six-six-six? doesn't that offend you?"
but historical scholars aren't so sure about 666. per wikipedia:
While most manuscripts of the Bible read six-hundred-sixty-six or 666 for the Number of the Beast in Revelation 13:18, some early manuscripts contain 616 instead. Codex Ephraemi Rescriptus (5th century) and Papyrus 115 (3rd or 4th century). Irenaeus knew about the reading 616, but according to Metzger, Bruce M. (1971) A Textual Commentary on the Greek New Testament, New York: United Bible Societies. ISBN 3-438-06010-8, p. 751, Irenaeus "says that 666 'is found in all good and ancient copies,' and is 'attested by those who had themselves seen John face to face.'" The minuscule manuscript 2344 (11th century) identifies the Number of the Beast as 665.
In May 2005 it was reported that scholars at Oxford University using advanced imaging techniques had been able to read previously illegible portions of an early (third century) version of the Book of Revelation, part of its Oxyrhynchus collection of papyri. The fragment gives the Number of the Beast as 616. Scholars now believe the number in question has very little to do with the devil. It was actually a complicated numerical riddle in Greek, meant to represent someone's name. "It's a number puzzle — the majority opinion seems to be that it refers to [the Roman emperor] Nero." Alternative spellings of his name, Neron and Nero, can explain the presence of both 666 and 616 in the records, since dropping the last 'n' changes its numerical value from 666 to 616 ('n' representing 50).¶