Though similar to the Piggy Bank item, the Swallowed Penny is not limited to just pennies.
Unlike Piggy Bank, playing as Keeper with this trinket always drops a coin, allowing semi-infinite life.
A trick for farming money when playing as Keeper is to find a room with some static hazard like spikes and taking damage, causing a coin to appear. A penny will only refill life while a nickel will give four cents, and a dime will give nine cents.
This trick can also be used to survive multiple uses of a sacrifice room, healing with the dropped coin between each sacrifice.
As it can now give 0 coins when playing as the Keeper, this no longer works. When combined with Piggy Bank, money will still be gained on average, but it is no longer guaranteed. Using IV Bag as the source of damage increases the average amount of coins per damage to 1.5, meaning that it will usually generate a pool of coins that can be used during an unlucky break or for later healing.
Synergies
Mom's Box: Spawns two coins on hit, multiple copies don't stack.
Golden Trinket / Mom's Box: Each copy spawns one more coin on getting hit with a limit of 5 coins.
When held by 
Keeper and 
Tainted Keeper, spawns 0-1 coins.
Special Shopkeepers, and 
Downpour.
Piggy Bank item, the Swallowed Penny is not limited to just pennies.
Unlike 
IV Bag as the source of damage increases the average amount of coins per damage to 1.5, meaning that it will usually generate a pool of coins that can be used during an unlucky break or for later healing.
Mom's Box: Spawns two coins on hit, multiple copies don't stack.
Golden Trinket / 
Penny Trinkets 
