All mobs drop coins, you can use them with NPCs to get items. The first teir of coin is the copper coin, then the silver coins, then the gold coin, and finally the platinum coin.
If you want to turn a higher teir coin into a lower one use the following recipe
9 copper coins = 1 pile of copper coins
9 piles = 1 silver coin
9 silver coins = 1 pile of silver coins ect.
If you want to turn a higher teir coin into a lower one use the following recipe,
Merchant - Right
Nurse - Middle
Demolitionist - Left
Copper Pickaxe: 5 silver
Copper Sword: 4 silver
Torch: 50 copper
Lesser Healing Potion: 3 silver
To summon, make a summoner item by putting 5 silver coin piles on a crafting table. Then hold the summoner item to summon the merchant.
Musketball : 7 copper coins
Flintlock Pistol : 5 gold coins
Minishark : 35 gold coins
To summon, make a summoner item by putting 1 boomstick (gotten from chest) in a crafting table. Then hold the summoner item to summon the arms dealer.
Lesser Healing Potion: 5 copper
To summon, combine a merchant summoner and a life crystal
Grenade: 64 copper
Bomb: 3 silver
Dynamite: 20 silver
To summon, combine a merchant summoner and a grenade (can be obtained from underground chest)
To summon, put 5 Gold Bars and one Ruby on a crafting table to get a Golden Crown. Place the Golden Crown and 5 gel in a crafting space to get the Slime Crown. Then hold the item in your hand and it will spawn.
The only way to spawn Skeletron at this time is to find a Dungeon (found in plains) and it will spawn there. In the future it may be spawn with a NPC or when you've gone to deep.
To summon the Eye of Cthulhu put six lens (from Demon Eyes) on a table to get a suspicious looking eye. Then hold the item in your hand and it will spawn. Drops 3-4 gold coins, 30-90 crimtane ore, and 5-15 healing potions
To summon, put a vertebra (from any crimson biome mob) and a Vicious Mushroom (block found in the Crimson biome) on a table to get a Bloody spine. Hold the item for it too spawn. Drops 5 gold coins , 40-90 crimtane ore, and 5-15 healing potions