Note: Tainted items are most likely to be only passive. just a note for you crazy goobers

Bob's Brain > Bob's Brainfreeze
"Kick it and run!"
Spawns a kickable familiar that explodes on contact with an enemy.

Steam Sale > Liquidation Sale
"Everything must go!"
A random, unpurchased item from a floor's shop will appear in the next floor's shop at a discounted price.

Judas' Shadow > Judas' Lament
"Dark retaliation"
Activate Dark Arts on damage. While held, Dark Arts' effect will apply the damage boost normally received by Dark Judas/Tainted Judas.

TMTRAINER > 4 4 Hy
"Reroll... or ZZAZZ"
Item pedestals rotate between a normal and a glitched item.

Gnawed Leaf > Smashed Leaf
"Unstoppable"
After moving for a short time, Isaacs face gets visually cracked open and he will gain a tears up shooting extra inaccurate blood tears until he stops moving.

Loki's Horns > Loki's Drift
"Split Tears"
Gives a random chance for projectiles to split into two.

Keeper's Box > Miser's Crate
"Portable Special Shop"
Spawns a trinket or collectible that cost 20-40 coins

BBF > Big Brave Drake
"Dragon Buddy"
Makes a firewave if it hits an enemy,similar to a Dragonfly!

Maw of the Void > Maw of the Abyss
"Consume them from far away"
Adds a secondary charged attack that shoots a brimstone ring in the player's direction!

Random question, but what is the non-tainted item equivalent of Atlas? Nevermind, found it by accident.
Also possibly a bug, but if you're using EID and have the mod names enabled, it displays as "The Shovel" for me, unless that's intended. It might be taking a different mod as precedence but I don't know.

Ideas for some Tainted items (incoming wall of text):

The Inner Eye --> The Outer Eye
"Triple Shot?"
Spawns a floating eye familiar that hovers in front of Isaac that fires a triple shot (it would fire faster than baseline triple shot as if you had the item, because triple shot with certain other items can wipe rooms e.g. Brimstone)

Spelunker Hat --> Spelunker's Whip
"Double tap whip"
Double-tap the firing keys in a direction to lash a whip forwards. This whip attack damages enemies, knocks enemies back, and can grab pickups (similar to Boomerang). Has a short cooldown between whip attacks.

Holy Water --> Unholy Water
"Maim and burn"
Spawns an unholy water familiar. Firing in a direction sends the familiar forward. On collision with an enemy, the container explodes, releasing tons of demonic red flames and Brimstone beams outward that deal heavy damage to enemies hit and burns them. BFFs increases the number of Brimstone beams released. Forgotten Lullaby increases the number of red flames released.

Lost Contact --> Found Contact
"Absorbent tears"
Your tears gain additional damage as they pass through enemy tears.

Contract From Below --> Contract From Above
"Gifts from above, but at what cost?"
No longer spawns pickups as room drops. Angel deal chance is automatically set to 100% for the remainder of the run (same as Eucharist). Entering the Angel room on that floor spawns all of the pickups that would've spawned from rooms but doubled.
- Note: My intent for this one is that its the opposite of Contract From Below (duh) in that it spawns no pickups, but spawns all of them doubled in the Angel room. You get no pickups for the floor until the Angel room, but you also get the Angel room for free because 100% chance. Think of it as a double-edged Eucharist.

Blue Map --> Tainted Map
"Ultra Secrets"
Reveals the Ultra Secret room on the current and all future floors. Bombing a wall thrice that is adjacent to the Ultra Secret Room spawns a red room on said wall.
- Note: Yes I know an item like this exists in Repentance+ (because I also use that mod), but it fits here thematically. This one costs three bombs instead of a trinket, so I hope you got a lot of bombs and/or you got this item early.

Hot Bombs --> Cold Bombs
"Freezing blast +5 bombs"
Bombs slow enemies that get close to them. If a normal enemy is damaged/killed by your bombs, they are instantly killed and frozen. Bosses are slowed for a short duration when damaged by your bombs.

Azazel's Rage > Benighted Rage
"Ancient Power"
Fires a giant brimstone ball instead of a laser!

Lazarus' Rags > Enigma Rags
"Eternal Life?"
Upon death,the player respawns as Tainted Lazarus!

Esau Jr. > Dark Esau Jr.
"Jealous Brother"
One-time use active,Spawns a Dark Esau Jr. familiar upon use,which will attempt to kill enemies and the player while doubling item pedestals in Treasure Rooms(Similar to Damocles)!

Cracked Orb > Shattered Orb
"Pieces of Knowledge"
Adds a chance to shoot a Orb Shard projectile that triggers the effect of Cracked Orb if it hits an enemy.

Samson's Lock > Savage Lock
"Berserker's Rage"
Has a 10% chance to trigger Berserk when killing an enemy

Beth's Faith > Zealot's Faith
"Portable Item summoner"
Spawns 1-3 lemegeton wisps upon entering a new floor.

brimstone->sacred beam!
replaces your tears with a charged up crack the sky esq shot that pierces enemies and moves slowly

Harlequin Baby > Baby Harlequin
"Juggling friend"
Absorbs enemy shots like Sworn Protector and juggles them. When it is juggling 3 shots, tosses them at the nearest enemy.

Fruity Plum - Sour Plum "She's gone bad!"

Slightly larger, slower version of baby plum. it will painstakingly crawl towards enemies slowly, before spewing out tons of low damage tears. doing this pushes her back very lightly, but lets her do it again soon after.

Technology > Nature
"Whip tears"
Tears switches to tainted Lilith like "attack mode", but instead of "Gello" it's grass vines that sticks to an enemies and damaging them by ticks. And there can be multiple vines.
They need to be charged like Forgotten's throwing bone.

Godhead > Devilhead
"Satan blood"
Now Isaac shoots with little brimstone homing stars without aura. With each next enemy on the way it grows bigger, like "Eye of belial" but 10 times.