We cannot know for sure what it was that he wrote on the ground. According to some there were supposedly a few old manuscripts that add the portion of "their sins who accused her, and the sins of all men" but there is no verification of this available today.
Amen P.R.
This is well said and I agree. I wanted to see how many different answers I might get. I've heard a few people preach on this and they've said some silly stuff about what they think He wrote. They were trying to make something out of nothing.