Coins

Coins serve as the currency for Shops, the payment for the Donation Machine in Shops, and payment for Slot Machines, Fortune Telling Machines, Shell Games, Beggars, Shop Restock Machines, and Crane Games. They are also consumed to use a few activated items (most notably Magic Fingers and Portable Slot). Keeper can also pick up coins to refill his health.

Coin	ID	Worth	Additional Info	Unlock Method	Probability
Penny	5.20.1	1	Humbleing Bundle and Equality! can replace this with a Double Penny.	Always unlocked	92.63% 91.717%
Nickel	5.20.2	5		Always unlocked	4.68% 4.875%
Dime	5.20.3	10		Always unlocked	1.00% 0.995%
Double Penny	5.20.4	2	Can't spawn naturally as a normal coin. Can only be found by using certain items or encountering certain room layouts.	Always unlocked	0.00%
Lucky Penny	5.20.5	1	Increases luck by 1 when picked up. Humbleing Bundle and Equality! will not double this. Bumbo and Bum Friend will pick this up, but will not give Isaac any luck.	Donate 2 Coins to the Greed Donation Machine	0.94% 0.985%
Sticky Nickel	5.20.6	5	Will flip upside-down when Isaac tries to collect it. Afterwards, it will get stuck on the ground, unable to be picked up unless blasted off the ground with an explosion. Bumbo and Bum Friend are unable to pick this up; they instead hover over them, attempting to pick them up. A Level 4 Bumbo will drop bombs in an attempt to pick it up.	Complete the Boss Rush as Keeper	0.25% 0.926%
Golden Penny	5.20.7	1	Spawns another golden penny in a random spot in the same room after being picked up, but has a small but increasing chance of disappearing instead.	Defeat Mega Satan as Tainted Keeper	0.50%

Notes

Upon pickup, golden pennies have a $0.9 8^{(n - 1)}$ chance to spawn another golden penny, where $n$ represents the number of times it has been picked up.
Unlocking Sticky Nickels is strictly speaking a downgrade, as they replace normal nickels, though the sticky ones are rare enough to not make a severe difference.
If Lucky Pennies have not been unlocked, they will always be replaced by a Penny in any room layouts containing them.
- Golden Pennies will also be replaced by random Coin pickup if not unlocked yet.
Booster Pack#5 added a green glow to Lucky Pennies, making them easier to spot. This was from this Steam Workshop mod by piber20.
Ultra Greed and his statue cannot pick up Sticky Nickels until they have been unstuck by an explosion, thus negating some difficulty from the fight.
- During the Ultra Greed fight, if a Sticky Nickel drops, Ultra Greed will be continuously drawn to it.
- Bumbino also has this behaviour.
Sticky Nickels visibly stick to the ground upon spawning, making them easier to distinguish from ordinary nickels.
Sack Head can cause a Golden Penny to spawn a sack instead of another Golden Penny.
The probabilities in the table refer to the kind of coin that is generated by the game when a coin is spawned with a sub-type of 0 (i.e. a random coin).

Cain - Hold 55 Coins at one time.
It's the Key - Defeat The Lamb without taking Hearts, Coins, or Bombs through the entire run.
Golden Razor - In a single run, obtain 99 Coins, then spend all of them.