From MTG Salvation Wiki
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.
It works by triggering when a player plays a spell with ripple, the triggered ability then lets a player reveal a given number of cards from the top of his or her library and play all cards revealed with the same name as the card with the triggered the ability without paying their mana cost. 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.
- When you play a spell with ripple, the ripple ability will resolve before the spell does.
- If you choose to reveal cards, you may play any or all of the revealed cards with the same name as the spell. Any cards you don't play, 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 play, follow all the normal steps for playing it, though you won't have to pay its mana cost. Any additional costs are paid as normal. All cards played this way go on the stack on top of the original spell, then all abilities that trigger when you play the new spells (including their ripple abilities) go on the stack on top of them.
- After you play 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 play a spell during a ripple ability's resolution; after the ripple ability resolves, the active player receives priority.