Temporal Aperture

{5}, {T}: Shuffle your library, then reveal the top card. Until end of turn, for as long as that card remains on top of your library, play with the top card of your library revealed and you may play that card without paying its mana cost. (If it has X in its mana cost, X is 0.)

World Championship Decks 1999

Urza's Saga

o5, oc
T: Shuffle your library and reveal the top card. Until end of turn, as long as that card remains on top of your library, you may play the card as though it were in your hand without paying its casting cost. (If the spell has o
X in its casting cost, X is 0.)

If your library is shuffled or that card otherwise leaves the top of your library, the effect ends.

The card is removed from the top of the library when you play it, just like a card is removed from your hand when you play it.

While you do not have to pay the mana cost, you do have to pay any other costs described in the text that are paid when playing the card. If the cost in the card text is optional, such as with Buyback, you can optionally pay that cost.