Ripple

Ripple is a triggered ability that first appeared in Coldsnap. It so far has appeared on two instants, a sorcery, an aura and a creature, as well as being given to all spells you control by the artifact Thrumming Stone.

Description
When a player casts a spell with ripple, the triggered ability allows the player reveal a given number of cards from the top of their library and cast all cards that were revealed this way with the same name as the spell without paying their mana cost. The rest of the cards are then put on the bottom of that player's library. So far, all the cards with Ripple have a Ripple value of four, but if the mechanic was revisited in future sets this could be altered.

Rulings

 * When you cast a spell with ripple, the ripple ability will resolve before the spell does.
 * If you choose to reveal cards, you may cast any or all of the revealed cards with the same name as the spell. Any cards you don't cast, including those with the same name, will be put on the bottom of your library in any order.
 * For each revealed card you choose to cast, follow all the normal steps for casting it, though you won't have to pay its mana cost. Any additional costs are paid as normal. All cards casted this way go on the stack on top of the original spell, then all abilities that trigger when you cast the new spells (including their ripple abilities) go on the stack on top of them.
 * After you cast the original spell with ripple, ripple's triggered ability goes on the stack and you receive priority. But you don't receive priority after you cast a spell during a ripple ability's resolution; after the ripple ability resolves, the active player receives priority.

Trivia
Thrumming Stone famously combos with Relentless Rats or other relentless creatures. Each time that a Relentless Rats is revealed and cast, it will trigger another Ripple 4. It is not uncommon for a Rats deck to run 20 Relentless Rats in the deck, so a Thrumming Stone, followed by a Relentless Rats could flush an army of Relentless Rats onto the battlefield each giving the other a static +1/+1 bonus.