The answers previous to mine are all good. The consensus of those answers is that we do not know what Jesus wrote on the ground in the incident of the woman caught in adultery, and I agree with the consensus.
Having heard a sermon recently based on this incident, however, I gained a little insight which led me to conclude the following:
It's not what Jesus wrote on the ground that matters. What matters is that He wrote on the ground. In other words, the content of Jesus' writing isn't nearly as important as His act of writing.
Prior to hearing the sermon I was well aware of how rabbinic tradition in Jesus' day was considered exceedingly important, and in some cases even more important than the sacred Torah, especially to the party of the Pharisees. As Rod Reynolds points out in his article "Did Jesus Break the Sabbath?",
To understand what is at issue in these accounts [of Jesus being accused of breaking the Sabbath], it is helpful to understand something of the rabbinical tradition that lay behind the Sabbath-breaking charges leveled against Jesus and His disciples. The Pharisaic tradition, by Jesus' day, had developed into an array of petty rules having to do with the minutiae of the law. It focused on physical works that had little to do with the spirit and intent of the law—and which, in fact, often violated the law (Matthew 15:1–9; Mark 7:1–13; John 7:19; Galatians 6:13).
The scribes among the Pharisees created and transmitted the Pharisaic rabbinical traditions. The body of traditional law that they formulated, called the Halakah (preserved in the Mishnah), is extra-biblical. Although authoritative for Jews who follow Pharisaic tradition, much of the Halakah is not directly supported by Scripture, but is intended as a "hedge" about the law, to prevent any possibility of its being broken.
Ironically, in an attempt to ensure their law-keeping by putting a "hedge" about the law, the Pharisees were breaking the law, for God had said: "You shall not add to the word which I command you, nor take anything from it, that you may keep the commandments of the Lord your God which I command you" (Deuteronomy 4:2; also 12:32). By adding the weight of their tradition to the law of God, they bound "heavy burdens, hard to bear, and lay them on men's shoulders" (Matthew 23:4).
The Pharisees placed the authority of their traditions above that of Scripture itself, thus going against the word of God. Scripture scholar Joachim Jeremias affirms that for the Pharisees, the oral tradition was "above the Torah," and that the esoteric writings containing scribal teachings were regarded as inspired and surpassing the canonical books "in value and sanctity" (Jerusalem in the Time of Jesus, pp. 236, 238–239). Alfred Edersheim also points out that traditional law was of "even greater obligation than Scripture itself" (The Life and Times of Jesus the Messiah, Book I, 1.98).
What was the nature of these traditional ordinances? "The Halakah indicated with the most minute and painful punctiliousness [attention to detail] every legal ordinance as to outward observance.… But beyond this it left the inner man, the spring of actions, untouched." Echoing what Jesus said (Mark 7:5–13), Edersheim continues: "Israel had made void the Law by its traditions. Under a load of outward ordinances and observances its spirit had been crushed" (Book I, 1.106, 1.108).
The sometimes-absurd contradictions within Pharisaic law are especially apparent in the rules of Sabbath observance.
Mr. Reynolds quotes extensively from Albert Edersheim's seminal work, The Life and Times of Jesus Christ, Volumes I and II, and if you are curious as to how punctilious the Pharisees were regarding the "keeping of the Sabbath," consult Edersheim (though for the time being, simply read Rod Reynolds' article).
The preacher of the sermon I alluded to earlier said that one of the rabbinic traditions regarding the Sabbath was that a pious Pharisee was allowed to write only so many letters--not words, but letters--on the Sabbath. While the preacher did not give a source for this tidbit, it makes sense to me, since the Pharisees had regulations about how large a piece of paper (papyrus) one was allowed to carry on the Sabbath. For example, a scrap of paper large enough to be converted into a note or a wrapper was not allowed!
All this to say, Jesus may have engaged in a situational irony in what He did by writing on the dusty ground with His finger. Since the scribes and Pharisees had a tradition regarding how many Hebrew letters one was allowed to write on the Sabbath, Jesus would respect that "law" by writing on the ground, and not paper. Stay with me now! Instead of asking for a clay tablet or a piece of paper and an inkpot, Jesus may have been engaging in a bit of irony by saying in effect,
"By writing on the ground, and not on paper, I'll respect your silly tradition which has no basis in the Law of Moses, although perhaps after I write on the ground you'll come up with a law which says one may not even write on the ground on the Sabbath! What I will not respect, however, is the way you treat this woman caught in adultery, asking if she, but not her partner in the sin, should be stoned to death. Don't put me to the test regarding adultery if you have no intention of carrying out that which is spelled out clearly in the Law of Moses; namely, ". . . the adulterer and the adulteress shall surely be put to death," and "If a man is found lying with a married woman, then both of them shall die" (Leviticus 20:10; Deuteronomy 22:22).
In other words, the woman's accusers were punctilious about obeying legalistic oral traditions having no basis in the Law of Moses, but they ignored the crystal-clear commandment concerning adultery by singling out the adulteress but not the adulterer.
That is why what Jesus did by writing is more important than what he wrote. Through His actions He was demonstrating how foolish and hypocritical the accusatory scribes and Pharisees were by inventing their own laws but not obeying God's revealed Law.
Ironic, isn't it?!