The Jar

Effects

Isaac can store up to 4 extra Red Hearts in The Jar if there are no remaining empty Heart Containers.
When used, all Red Hearts in the jar are dropped on the floor, rather than directly consuming them.

Red hearts are the only type of heart that can be stored. Blended Hearts do not count.
If used in the two Super Secret Rooms that contain nothing but a Black Heart or Eternal Heart (Room ID's 1 and 6), it will spawn those types of hearts instead.
If a whole red heart is picked up and Isaac only heals half a heart container, the other half is placed in the jar, indicated by the jar number increasing by one. Despite the jar indicating that it picked up a whole red heart in this fashion, it will only drop half a red heart on use. When picking up a double red heart, the jar will also store the remainder not healed in the same way.
Co-op babies, except Magnet Baby, cannot add hearts to the jar, as they are unable to interact with pickups.
The short-lived half red hearts dropped by (Tainted Magdalene) Tainted Magdalene can be picked up and stored.

(Added in Repentance) Book of Virtues: Consumes stored hearts to spawn heart wisps, rather than releasing heart pickups. Heart Wisps have a chance to drop a half red heart when destroyed.
(Removed in Repentance) Humbling Bundle / Maggy's Bow: Spawn double the hearts put into the jar, allowing for infinite health to be spawned.

(Broken Remote) Broken Remote: Allows for teleportation with no recharge; however, it drops the contained red hearts in the room it was used.
(Added in Afterbirth) Car Battery: The Jar acts as usual, and doesn't double the hearts dropped upon activation.
(Added in Afterbirth †) Schoolbag: The player cannot pick up extra hearts while The Jar is in the inactive slot.
(Added in Repentance) Shard of Glass: Storing a heart in the jar does not stop the bleeding effect, despite playing the "heart collected" sound effect.