Isaac is completely invisible, however you can still see the light effect surrounding him.

In The Binding of Isaac: Rebirth, the seed determines every item, enemy, floor layout and room in the run. You won't be unable to unlock secrets while playing seeded runs, however they allow you to share the seed with your friend and play the same run together.
Rebirth has over 4 BILLION possible seed combinations, however there are 50+ pre-assigned seeds which have unique effects on the game. This page highlights all the secret seed codes which the developers have added to the game and what each seed does.
Note: Seeds are all 8 characters long and all alphanumeric characters are allowed except the letters I, O, U and V, because they look too much like other characters.
Isaac is completely invisible, however you can still see the light effect surrounding him.

Gives you a hard timer of 30 minutes to beat the game. After 29:45 a timer starts ticking and at 30:00 you will take damage every second.
Music is permanently sped up.
Music is permanently slowed down.
Endless run, where every floor is the basement! Due to how the item spawning mechanics work, after the item pool is exhausted, every item will become the Breakfast health up item.

Turns all enemies invisible.

Increased gore effects when enemies die.
Applies a filter to the game which makes it look like a CRT television screen.

Taking any damage instantly kills you.
Isaac leaves a trail of poop behind him as he walks.
All pickups are now covered in a colour similar to the floor, making them harder to see.

The player sprite is removed and replaced with a brown coloured shadow which is similar to the floor colour.

Turns every eligible enemy into a champion version.

Gives Isaac a Santa hat! (Same effect as 'H0H0 H0H0' - Added in v1.04)

All enemies have a permanent charm effect and will attack each other if hostile.

Enemies respawn when you leave the room and come back.
All enemies have a permanent concussion effect, making them walk around the room dazed.

Gives curse of the unknown on every floor.
Isaac takes damage if he stops moving.
Inverts your movement and firing controls.
Ingame text is re-arranged.

All Tarot Cards when picked up are face down so their effect is always unknown.
Does what it says on the tin - Sound effects are replaced with fart noises.
All enemies have a permanent fear effect.

All enemies have a fear effect which turns on and off every few seconds, causing enemies to run away sometimes.

Starts you with 69 coins and makes it so you have to purchase all of your items and pickups.

All damage does 1 full heart for every floor.
The lower your health is, the slower the music is.
Gives Isaac a Santa hat! (Same effect as 'CHRS TMAS' - Added in v1.04)

Turns Isaac's sprite grey coloured like the twitch.tv Kappa emote.

Pickups mimic player movement.
Removes all parts of the UI overlay, including map, consumable count, health etc.
Causes the Player 2 coop baby to be invincible.
Music is permanently slowed way way down (More than the ANDA NTE seed).
All pill text (names and descriptions) are replaced with ???.

Enemies and bosses cannot be killed, all doors remain always open and bosses have their health slowly drained.
Music is permanently sped up a lot (More than the ALLE GR0 seed).
Turns Isaac into a jet black shadow.

Music speed changes based on how fast you are walking.
Tears have a slight glow around them (Hardly noticable unless you have Curse of Darkness).

Isaac has no face!

Every 45 seconds a ticking sound will start and after 5 seconds Isaac takes damage. A reference to when Edmund stated there would be a Spelunky ghost-like mechanic in the game and it was taken out of context by some of the community.
Gives Isaac a teeny-tiny head.

Gives Isaac a HUGE head.

Isaac, Enemies, Consumables and Items all have a brown filter and blend in with the floor (Similar to CAM0 K1DD / CAM0 F0ES).
