Broken Shovel: Prevents the Mom's Foot from appearing for two rooms (empty or not), or for two waves of a Challenge Room or the Boss Rush, essentially removing the stomping attacks for the entire duration of the Boss Rush
Crooked Penny: Splits the 50% chance to succeed into a 25% chance to quadruple all items and a 25% chance to duplicate all items while consuming the originals; the latter will make Devil deal and Shop items free.
The 25% chance to duplicate all items while consuming the originals now generates 2 pennies instead.
Sulfur: Will count as giving double Brimstone, giving extra damage and a x1.2 damage multiplier for the current room.
Void: Doubles effects of all absorbed activated items. Absorbing passives will not give extra stat ups.
Wooden Nickel: 25% chance to spawn nothing, 50% chance to spawn one coin, and 25% chance to spawn two coins.
Berserk!: Will only increase rage duration if no enemies are killed in the first half, effectively only affecting bosses that can't spawn enemies.
Clicker: If the Car Battery is removed by the first activation, a second activation will not happen. Two activations in a row is mostly detrimental, removing two items and possibly cycling through a character that lowers health.
Spindown Dice: Lowers ID by 2, good for some items, bad for others.
Dark Arts: Doubles the duration, giving more time to walk through more enemies, if you end the effect before the end of the first second, the item will reactivate itself; allowing to target the same enemies twice.
D4/D100: Instantly re-rolls Isaac's items twice, which doubles the speed at which the item pools are exhausted and transformations are achieved.
As items are returned back into the item pool when re-rolled, the item pools aren't exhausted at all. In addition, you need to actively be holding a transformation's respective items to retain them now, so using the double re-roll to gain multiple transformations is no longer possible.
The D6/D8/D12/D20: Re-rolls twice instantly without any benefits. The D6 depletes item pools at double the speed.
D7: Doubles the amount of enemies summoned, but does not give double rewards.