Nineveh
Nineveh, at the start of the 8th century B.C., was a large metropolis in the still expanding Assyrian Empire. Various Bible commentaries state the city's population, at the time of Jonah, was anywhere from 120,000 people to 600,000 or more.
Research carried out on ancient populations suggests that the pagan city, in the fifty-six years before its destruction in 612 B.C., was the most populated area in the world (4,000 Years of Urban Growth: An Historical Census).
The city's wicked behavior drew God's attention and cried out for his judgment (Jonah 1:1 - 2). The Lord decides, however, to extend some mercy to the city. He sends the minor prophet Jonah to warn Nineveh of its sinful ways and impending destruction (3:4).
Jonah, although God had to convince him to fulfill his mission, eventually warns Nineveh that its judgment was rapidly approaching (Jonah 4:4). The city's immediate response was to cause everyone, including the animals, to begin to fast. The king of Nineveh, who also fasted, even commanded the people to repent from their evil ways in the hope that they might receive mercy (3:5 - 9).
The extraordinary response of those in Nineveh, referenced by Jesus himself (Matthew 12:41), led to God extended more mercy to the city by deciding not to overturn it!
Saved from certain death
King David was a grateful and frequent recipient of God's mercy, writing about in at least 38 Psalms. In one Psalm in particular, number 136, he praises the Lord's merciful acts in each of its twenty-six verses!
David, after he lusted after a married woman named Bathsheba, not only committed adultery with her but also sought to cover up his sin by arranging for the death of her husband Uriah (2Samuel 11, 12). God's law required those committing such acts to be punished with the death penalty (Exodus 21:12 - 14, Leviticus 20:10, etc.).
The prophet Nathan is sent to confront the king with his great sins. After repenting of what he had done, God extended mercy to David by having Nathan tell him, "The Lord also has put away your sin; you shall not die" (2Samuel 12:13). David was saved from certain death because he quickly admitted his sin and the Lord's mercy took into account his heart of repentance (see Psalm 51).
Jerusalem spared destruction
David required another big dose of mercy after he committed the sin of taking a census of Israel's fighting men. After confronted about his sin, the king chooses as his punishment a three-day long deadly epidemic upon the entire land.
God, after a death angel kills 70,000 Israelites, halts the slaughter before it enters Jerusalem (2Samuel 24). David, seeing the angel, pleads for God's mercy that no more lives be lost. The plague is permanently stopped after the king builds an altar and offers sacrifices upon it (verse 25).