Iron Ingot

Iron ingots are versatile metal ingots used extensively in crafting.

Smelting
Smelting iron ore is the most common way to obtain iron ingots.

Iron golems
Iron golems drop 3-5 iron ingots upon death, regardless of the way they die. The looting enchantment does not increase the amount of iron ingots dropped.

Zombies
Zombies, husks, and zombie villagers have a 2.5% chance of dropping either an iron ingot, carrot, or potato when killed by a player or tamed wolf. This is increased by 1% per level of looting. This gives iron ingots the following chances of dropping:
 * $1/120$ (about 0.83%)
 * $11/600$ (about 1.83%) with Looting I
 * $17/600$ (about 2.83%) with Looting II
 * $23/600$ (about 3.83%) with Looting III

Trading
Apprentice-level Armorer, Toolsmith, and Weaponsmith villagers buy 4 iron ingots for an emerald as their third trade.

Apprentice-level Armorer villagers have a 50% chance to buy 4 iron ingots for one emerald, and apprentice-level Toolsmith and Weaponsmith villagers will always buy 4 iron ingots for one emerald.

Armorer, toolsmith, and weaponsmith villagers buy 7-9 iron ingots for one emerald as part of their second tier trades.

Repairing
Items which can be repaired with iron ingots in an anvil;

Healing iron golems
$$, an iron ingot on an iron golem restores its health by.

Beacons
Iron ingots can be used to select powers from a beacon. The player must select one of the available powers, and then insert an ingot in the item slot.

An iron ingot can be substituted for an emerald, a gold ingot, or a diamond.

Trivia

 * The iron ingot is the most versatile item in the game, being used in 34 crafting recipes. The second most versatile item are planks with 29 recipes.
 * To craft each recipe once, you would need 153 ingots, including 27 for the iron blocks in an anvil, and one for the tripwire hook in a crossbow.