8 Inch Nails

Effects

Does not grant penetrative shots, despite the appearance of Isaac's tears.
Any item that normally colors Isaac's tears red will cause the nails to become bloody.

(Blister) Blister: The knockback from Blister is stronger than from 8 Inch Nails, and knockback boosts do not stack.
Brimstone: The beam gains knockback.
Dr. Fetus / Epic Fetus / Ipecac: Explosions also gain increased knockback.
The Ludovico Technique: The controlled tear gains knockback.
Mom's Knife: The knife gains knockback.
Piercing tears: Allows tears to knock enemies back again.
Tech.5 / Tech X / Technology / Technology 2: The lasers gain knockback. Especially powerful with Tech X, as the knockback helps keeps enemies within the ring for longer.
(Added in Afterbirth †) Trisagion: The blasts gain knockback.