The Battery, this synergy allows Isaac to (considering he entered the Boss Rush room with a full charge) essentially remove the feet 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.
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.
Sulfur - Counts as double activation for the current room.