Enemies that make contact with Isaac take damage equal to the number of coins he has.
The damage is 3.5+0.2 * the number of Isaac's coins. The damage caps at 23.30 with 99 coins.
Damage is dealt up to twice per second per enemy.
Notes
With Midas' Touch, some effects that create Poop may instead create Golden Poop (see Synergies for details).
If an enemy doesn't deal contact damage, Midas' Touch can safely be used on it to create extra Pennies. Flies, Bulb, Mulligan, and Host are the most common enemies this can be applied to.
If Isaac has 13 coins or more when using Midas' Touch on Mulligans, they will die and release flies, hurting Isaac.
With Skatole, Hives and Swarmers will constantly spawn black flies, meaning they can create endless amounts of coins.
As Mom's foot and Satan's feet don't deal contact damage, Midas' Touch can be used against them to great effect.
Spawning Multiplayer Babies while the player has Midas' Touch will give the Midas' Touch effects (freeze on touch + damage) to the baby itself. This can be abused, considering the baby's invincibility frames upon spawning:
Spawn the baby using the Enter key. It is invincible for a short period.
During this period, stay in contact with enemies to freeze them and damage them.
This can be used to gather 99 coins very quickly, and in turn, deal massive damage with Midas' Touch damage component.
Petrified enemies that are frozen will not drop any coins on death.
Midas' Touch can be a great item with Tainted Keeper, as you can run into enemies and get even more coins. Just be careful to not kill yourself.
Both Midas' Touch and E. Coli affect Tainted Forgotten's body, causing enemies that touch the body to freeze and turn into poop respectively.
If a violet champion monster is killed while it is gold-petrified, the trinket it drops is guaranteed to be a golden one. Rainbow champions still drop normal trinkets.
Epic Fetus + E. Coli: Launching a rocket at Isaac will turn the rocket into a golden poop, causing the explosion to break it automatically and spawning coins, which allows health to be exchanged for money.
Re-Lax: Spawned Poops have a chance to turn into Golden Poops. 25% at 0 luck and 100% at 8 luck.
Any item or card that grants Isaac temporary invincibility lets Isaac use Midas' Touch on any enemy without taking damage. This includes, but is not limited to:
This item is a reference to King Midas from Greek Mythology, who wished to be able to turn everything he touched with his hand into gold and ended up regretting his wish after turning his food to gold.
Pennies.
Keeper or 
Tainted Keeper, enemies will drop 0-1 pennies instead.
Poop may instead create 
Golden Poop (see Synergies for details).
Flies, 
Bulb, 
Mulligan, and 
Host are the most common enemies this can be applied to.
Skatole, 
Hives and 
Swarmers will constantly spawn black flies, meaning they can create endless amounts of coins.
As 
Mom's foot and 
Satan's feet don't deal contact damage, Midas' Touch can be used against them to great effect.
E. Coli affect 
Tainted Forgotten's body, causing enemies that touch the body to freeze and turn into poop respectively.
If a violet champion monster is killed while it is gold-petrified, the trinket it drops is guaranteed to be a golden one. Rainbow champions still drop normal trinkets.
A Card Against Humanity: Spawned Poops have a great chance to turn into Golden Poops. 10% at 0 luck and 100% at 18 luck.
Deep Pockets: Increases maximum contact damage. The damage caps at 203.3 damage with 999 coins.
Epic Fetus + 
Flush!: Touching poops made from enemies turns them into 
Counterfeit Penny.
The Poop: Using The Poop has a high chance to spawn Golden Poop.
VII - The Chariot/
VII - The Chariot?
A Pony/
White Pony
Book of Shadows/
Celtic Cross/
Empty Vessel/
Algiz/
The Polaroid/
Virgo
Breath of Life
Dull Razor
Holy Mantle
My Little Unicorn/
Unicorn Stump/Feels like I'm walking on Sunshine!
Aries (if Isaac's speed is greater than 0.85)
The Gamekid/Power Pill!
Ventricle Razor (briefly after exiting a portal)
Tooth and Nail
Mine Crafter: Has a chance to petrify the TNT block when placed, damaging Isaac in the process if he touches the golden TNT Block.
The Binding of Isaac: Rebirth
