The end of John 1:1 famously says, "...and the Word was God." The New World Translation of the Watchtower Society of the Jehovah's Witnesses translates this clause, "...and the Word was a god." They assert that this is the correct translation of the Greek because there is no Greek article before the word "God" (theos). In their view, John 1:1 doesn't teach that the Word is co-equal and co-eternal with the Father in divinity but that the Word is a lesser sort of divine being like an angel ("spirit creature" is their term).
Before looking at the Greek Grammar of John 1:1, it's important to see if the NWT is consistent with itself. When it comes to John 1:1, the JWs insist that a Greek word without an article (called "anarthrous") must be indefinite—that is, it must be translated with "a" or "an". But a quick scan of their translation of John 1 reveals that they do not follow their own translation principle in other places.
The Greek word for god (theos) occurs eight times in John 1:1-18 and only has an article two of those times (1:1b and 1:2). According to the NWT "rule", the word "god" should be translated "a god" every time it occurs without an article. In blatant violation of their own rule, John 1:1c is the only time they translate the anarthrous theos indefinitely.
1:1b (tov theov) -- "the Word was with God"
1:1c (theos) -- "the Word was a god"
1:2 (tov theov) -- "with God"
1:6 (theou) -- "of God"
1:12 (theou) -- "God's children" or "children of God"
1:13 (theou) -- "but from God"
1:18 (theov) -- "seen God"
1:18 (theos) -- "the god"
If that weren't enough, you can look at how "god" (theos) in used in the rest of the New Testament and find the same sort of translational tricks going on. The word theos occurs without an article 282 times in the Greek New Testament. Interestingly, there are only sixteen places where the NWT translates it as indefinite ("a god", "god", "gods", or "godly"). That means that they only followed their own rule 6% of the time.
In other words, they dishonestly choose to apply their "rule" only when it helps their agenda. This is a sure sign of blind ideology, not an honest and humble quest for truth. By all appearance, they decided that Jesus can't be fully equal with God, and then made up a Bible translation to try to support their beliefs.
An Honest Look at the Issue
Instead of twisting and rewriting Scripture to fit our beliefs, we must follow God's word wherever it leads us. This was a central doctrine of the Protestant Reformers and it is still a vital issue today. So how should we understand John 1:1?
Greek scholars mostly agree that theos in John 1:1c is neither definite nor indefinite but qualitative, which means that John is telling us about an attribute or characteristic of the Word. That may sound like it hurts the argument of orthodox Christians, but it actually strengthens the Christian position.
We've already looked at how the Jehovah's Witnesses translate John 1:1 when theos is taken to be indefinite. They only thing they succeed in doing is introducing polytheism into the Bible, which is, of course, nonsensical. But there is no reason for orthodox Christians to argue for theos to be taken as a definite noun in 1:1c. We would end up with something like this: "In the beginning was the Word, and the Word was with God [that is, the Father], and the Word was God [that is, the Father]." John is not suggesting that the Word is identical with God the Father. That would be an ancient heresy called Modalism, which taught that there is only one God but there are not three distinct persons within God. Modalism holds that there is one God who sometimes acts in the role of the Father, other times acts in the role of the Son, and other times acts in the role of Spirit.
Taking theos qualitatively, John is telling us that the Word is God in the same way that the Father is God. John's statement fits beautifully with the doctrine of the Trinity, because John is very subtly but very clearly describing the plurality of persons that exists within the one God. John is not telling us that the Word is identical with the Father, but he is telling us that the Word is God just as the Father is God. The Word is coeternal with the Father, but He is not the Father. He is coequal in divinity with the Father, but He is not the Father. The Word is God, but the Word is not the Father. The Word is not the Father, but He is God just as the Father is God. This is the glory of the Christian gospel: God Himself has come in the flesh to save us and to reveal the Father to us.
 Theos in John 1:1c is a predicate nominative that fits all the criteria for Colwell's Construction, which indicates that it should be taken qualitatively in this instance. For more information about Colwell's Construction, see Daniel B. Wallace, Greek Grammar, p. 262.