I find this utility mod useful when balancing enemies, armor, etc. Damage numbers are printed in a format (a : b), where a is a damage before reduction (what it would be like if you had no armor, enchantments and resistances), the second one is final damage recieved. Entitiy damage is yellow, player damage is dark red. If damage is magic, unblockable or absolute, it is printed as well.
Healing numbers may be annoying so you can turn them off with a command "/damagedebug heals".
This mod is server-side, meaning that you run it either in single player or on a multiplayer server.
