The Binding of Isaac wiki's got a [pretty comprehensive (if slightly generalized, and minorly inaccurate) list][1]. Remember that all cards are a single use item, so once they are used they vanish.

* **O The Fool:** Teleports the player to the starting room for that floor.
* **I The Magician:** Gives you the use of Spoon Bender (homing shots) until you leave the current room.
* **II The High Priestess:** Summon's Mom's foot stomp on a random enemy. Can also hit the player, and _will_ do so if there are no enemies in the room.
* **III The Empress:** Gives you the use of _Whore of Babylon_ until you leave the room. _Whore of Babylon_ is Eve's curse, but unlike her version, this just hurts you half a heart then increases your damage.
* **IV The Emperor:** Teleports the player to the Boss Room.
* **V The Hierophant:** Spawns two soul hearts.
* **VI The Lovers:** Spawns two hearts.
* **VII The Chariot:** Acts as a single-shot use of My Little Unicorn.
* **VIII Justice:** Spawns bombs, hearts, coins, and keys around the player.
* **IX The Hermit:** Teleports the player to the Mini-boss Room, or the shop if a mini-boss room is not available.
* **X Wheel of Fortune:** Spawns a Slot Machine.
* **XI Strength:** Makes the player bigger, increasing maximum health and damage until the player leaves the room.
* **XII The Hanged Man:** Gives you the use of Transcendence (flight over gaps, rocks and spikes) until you leave the current room.
* **XIII Death:** Gives you a single use of The Necronomicon (massive damage to all enemies in the room).
* **XIV The Tower:** Gives you a single use of Anarchist's Cookbook (spawns multiple bombs around the room at random).
* **XV The Devil:** Gives you a single use of Book of Belial (increased tear damage) until you leave the current room.
* **XVI Temperance:** Spawns a Blood Donation Machine.
* **XVII The Stars:** Teleports the player to the Item Room, or acts as Telepills if there is no item room (i.e. in The Womb and Sheol).
* **XVIII The Moon:** Teleports the player to the Secret Room.
* **XIX The Sun:** Completely heals the player, does damage to all enemies on screen and acts as a Treasure Map and Compass for the current level only.
* **XX Judgement:** Spawns a beggar. If you have no money, spawns 1 heart, coin, bomb and key instead.
* **XXI The World:** Acts as a Treasure Map and a Compass for the current level only.

Tarot Cards II, III, XVII, and XIX were added in a patch on November 1 2011.

  [1]: http://bindingofisaac.wikia.com/wiki/Tarot_Cards