A nun's Habit.
Isaac wears a Habit.
When Isaac is hit, 3 bars of the activated item charge are filled (as of v1.50 in the Unholy Edition, the refill is the amount of bars that would be gained upon clearing a room instead).
Book of Revelations/The Nail - Gives Isaac a spirit heart every time he takes two half-hearts of damage, essentially making him invincible. Acquiring The Wafer allows this combo to work even after The Womb.
Scapular - If you're willing to bring yourself down to half a heart left, and there is a reliable source of damage in the room (e.g. fire or spikes), then you can use your spacebar item as many times as you like. Gets interesting when you're playing as Isaac with the D6 or have otherwise acquired the dice.
The Dice - Can be used to reroll items over and over until you have the item you want, as long as you have enough health.
Book of Shadows/My Little Unicorn/The Gamekid - If Isaac has enough health to spare taking damage, items will charge after getting hit, granting near immortality for high health players (May prove more challenging on floors beyond The Depths/Necropolis) The latter two items allow a method of attack whilst invincible however are more difficult to exploit. The Gamekid although counteracts the longer charge time by providing a method to replenish red hearts.
Charm Of The Vampire - If there is a monster in the room that spawns enemies, (e.g. The Duke of Flies) the player can hurt him/herself to charge an item, then kill the re-spawning enemies in order to heal the damage done.
- A 'habit' is a set of religious garments worn by members of a religious order (generally known as monks and nuns but can have other names). The robes depicted in the game are similar to those of Franciscan nuns, either referencing the orders veneration of St Francis who was a martyr or the nuns who have died as martyrs over the centuries.
- Edmund McMillen has stated that he recognizes that items such as the Habit allow the player to become technically invincible, and says that it was intentional for this to be possible.