Grants a 50% chance to drop a random Coin, Bomb, Key, or Heart whenever Isaac takes damage.
The chances of the drops between each category are roughly equal.
Synergies
Fanny Pack: Having multiples of Fanny Pack neither increases the amount of drops, nor the drop chance.
Blood Oath: Each half-heart lost has a chance of spawning a pickup from the fanny pack.
Scapular: Allows for infinite pickups once Isaac is at half a red heart (or full red heart in Chapter 4 onward) by moving between rooms to reset Scapular's effect.
Placenta: Similar to Scapular, allows for unlimited pickups provided a disregard for time.
Sharp Plug: Each half a heart lost from the use of sharp plug has a chance of spawning a picking from the fanny pack.
Spindown Dice + Greed Mode: Turns into Humbleing Bundle when rerolled by Spindown Dice (which is not normally obtainable in Greed Mode) and this allows penny drops from each wave a chance of turning into double pennies.
Coin, 
Bomb, 
Key, or 
Heart whenever Isaac takes damage.
Fanny Pack: Having multiples of Fanny Pack neither increases the amount of drops, nor the drop chance.
Blood Oath: Each half-heart lost has a chance of spawning a pickup from the fanny pack.
Scapular: Allows for infinite pickups once Isaac is at half a red heart (or full red heart in Chapter 4 onward) by moving between rooms to reset Scapular's effect.
Placenta: Similar to Scapular, allows for unlimited pickups provided a disregard for time.
Sharp Plug: Each half a heart lost from the use of sharp plug has a chance of spawning a picking from the fanny pack.
Spindown Dice + Greed Mode: Turns into 
Humbleing Bundle when rerolled by Spindown Dice (which is not normally obtainable in Greed Mode) and this allows penny drops from each wave a chance of turning into double pennies.
The Binding of Isaac: Rebirth
