In the original text it's not actually written as "666" but as something like νρων κσρ in greek or, in modern hebrew letters, נרון קשר.

This is because, in those days, there was no separate numerical system and both cultures used letters of the alphabet as numbers, adding up various letters until the required total is reached (which is part of the logic behind numerology-based mystery cults like Kabbalah and so forth). A modern equivalent might use A as 1, B as 2 all the way up to J as 9, then K as 10, L as 100, M as 1000 and so forth.

The smart amongst you may have already noticed that the letters above spell "nron ksr" and "nrwn qsr" respectively. When the book of Revelation was written, it was done so in greek, as any literate writer in Israel of the day would have known Greek as well as Aramaic and old Hebrew. The writer may have written in Greek, however he was still thinking in Aramaic and wrote the name "nero ceasar" (or neron kaiser as it would have been in literate circles there, as they all spoke Greek rather than Latin), transliterated from aramaic to greek, as the "number of the beast". Aramaic, like Hebrew and most other semitic languages has no vowels, so the result would be the equivalent of NRWN QSR. With the transition to hindu-arabic numerals the transliteration lost its meaning and the total number was rendered simply as "666".

In some translations from greek to latin a mistake was made by the translator, who assumed that the text should say NRW QSR, resulting in some later texts having the number add up to 616.

So, to end it all, the whole "number of the beast" thing is actually a bit of historical curiosity now rather than a fundamental element of identification of some future "antichrist" figure. It's worth remembering that there is no mention of a single man named "antichrist" in the entire book of revelation. The entire book refers to events that took place around 69 AD, when the romans laid seige to Jerusalem. The beast of the sea was Nero, the beast of the land was the Jewish religious hierarchy, the "harlot" was that same hierarchy, the weeping merchants were the Jewish people and foreigners who traded in Jerusalem as it stood on the crossroads between east and west and so on and so forth. Those "end times" referred to throughout the new testament were a reference to the eventual sack of Jerusalem and the annexation of Israel as part of the Roman Empire, something anyone with a bit of foresight and brain could have predicted if they paid attention to the political motions of the day.

Basically the entire book refers to events in the past. It's over. Finito. Finished. We're living after the end of the book.

Marvellous. :)