Did you know that "stauros" -- the Greek word used in the New Testament for "cross" -- originally meant something like "stake"? The Jehovah's Witnesses are banking on you not knowing that. They believe that the whole church went far astray very early in its history, and is deceiving people about all kinds of things and hiding information and so forth. It's fairly boilerplate conspiracy-theory stuff. One thing they advance as proof is that "stauros" meant "stake" -- that Jesus was crucified/impaled on a vertical stake without a crossbar. Their artwork of the crucifixion has Jesus on a vertical stake, his hands pinned above his head rather than out to the sides. The traditional style of cross is considered something like a pagan import; "stauros" proves that the Jehovah's Witnesses are the true remnant of original Christianity, while our cross-shaped crosses prove we're the apostates, you see.
Besides the shape of the cross being irrelevant for our knowledge of Christ, it also ignores all kinds of information about the history of crucifixion as capital punishment, placing most of the weight of the argument on a single Greek word for a Roman form of punishment. Still, they claim that "stauros" is the key piece of knowledge. Many Christians have advanced arguments from the gospels, from ancient Roman sources, and from early Christian writings showing that the cross of Jesus was actually a cross (not that it much matters, other than keeping the Jehovah's Witnesses from sheep-stealing by advancing faulty arguments). I'd like to add one more piece of information: how the words for "cross" and "crucify" (you know, the "stauros" type words) were written in the oldest copies of the Gospels that we have.
One of the oldest manuscripts we have of the Gospels is a papyrus known as P75. Scholars date it somewhere around 175-225 A.D. -- fairly early in Christian history. In that papyrus, the scribe used special forms for the words "cross" and "crucify" (stauros and its cousin the verb stauroo - pardon the transliteration there, I'm not really going to switch over to another font just to get an omega). The Greek letter tau (second letter of stauros) looks roughly like our upper-case T; the Greek letter rho (fifth letter of stauros) looks roughly like our upper-case P. In several cases where the words "cross" or "crucify" appear in this ancient manuscript, the scribe has gone out of his way to drop out the "au" so that he can write the T and P shapes on top of each other, making something that physically resembles a person on a cross.
This is an enlargement of an actual photo of part of Luke 24:7 in the ancient papyrus P75. Luke 24:7 reads, "The Son of Man must be delivered into the hands of sinful men and be crucified and on the third day be raised again." The italicized part ("and be crucified", or "kai staurothenai") is the part shown in the image above. The original image can be found here, where if you patiently count down to the 25th line of text on that page and look towards the end of the line, you will see the original written by the hand of a scribe somewhere around the year 200 A.D. The word has a horizontal line written above it indicating the special form of the word. Notice how two letters have been dropped in order that the scribe can make the T and P shapes overlap to resemble a human figure on a cross.
There are several places in this ancient manuscript where either the noun for cross (stauros) or its matching verb for crucify are written with that special form. Other ancient manuscripts have been found from roughly the same time with the same special form of stauros: a form where the writing has been deliberately arranged to physically resemble a man on a cross. Given that there are other manuscripts from around the same time, it is really unlikely that these are the first people ever to write "stauros" that way; it was more likely becoming an accepted practice in Christian circles. But this would make it an accepted practice in Christian circles by around 200 A.D. to write "stauros" in a way that it was clear that, in this case, "stauros" itself did not mean a stake.