The Unmerciful Servant
This parable is Jesus' response to Peter's question: "Lord, how many times shall I forgive my brother who sins against me? Up to seven times?" Peter likely thought that the offer to forgive "up to seven times" was very generous on his part. But this parable puts such generosity in perspective.

The servant at the heart of this parable owes a truck-load of money to the king. And Jesus wants us to appreciate that we owe everything that we have to God. As the law says: "You may say to yourself, 'My power and the strength of my hands have produced this wealth for me.' But remember the Lord your God, for it is he who gives you the ability to produce wealth. (Deuteronomy 8:17,18)" As Jesus puts it: "From everyone who has been given much, much will be demanded; and from the one who has been entrusted with much, much more will be asked. (Luke 12:48)" Paul concurs:  "What do you have that you did not receive? And if you did receive it, why do you boast as though you did not? (1 Corinthians 4:7)." The principle here is related to what is commonly called "privilege". God is well aware of who we are, what we have, and what we can do. After all: he made us the way we are. So he isn't impressed by any of it. He doesn't judge us by any of that. Rather, he judges us by what we do with what we have  (see also this parable and this teaching ).
Matthew 18:23 (ESV) “Therefore the kingdom of heaven may be compared to a king who wished to settle accounts with his servants. 24 When he began to settle, one was brought to him who owed him [thirty billion dollars]. 25 And since he could not pay, his master ordered him to be sold, with his wife and children and all that he had, and payment to be made. 26 So the servant fell on his knees, imploring him, ‘Have patience with me, and I will pay you everything.’ 27 And out of pity for him, the master of that servant released him and forgave him the debt. 28 But when that same servant went out, he found one of his fellow servants who owed him a hundred [days' wages], and seizing him, he began to choke him, saying, ‘Pay what you owe.’ 29 So his fellow servant fell down and pleaded with him, ‘Have patience with me, and I will pay you.’ 30 He refused and went and put him in prison until he should pay the debt. 31 When his fellow servants saw what had taken place, they were greatly distressed, and they went and reported to their master all that had taken place. 32 Then his master summoned him and said to him, ‘You wicked servant! I forgave you all that debt because you pleaded with me. 33 And should not you have had mercy on your fellow servant, as I had mercy on you?’ 34 And in anger his master delivered him to the jailers, until he should pay all his debt. 35 So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart.”
Having been forgiven his immense debt, the servant now turns around and demands repayment of a (relatively tiny) debt owed to him. And the king isn't impressed. Once again, Jesus is teaching that how we deal with those around us has profound impact on how God will deal with us (see also this parable , this theme , and this teaching ).