Oracle Text

Counter target spell. You may cast a nonland card in your hand that shares a card type with that spell without paying its mana cost.

Card Rulings

1/22/2011 You cast the card in your hand as part of the resolution of Counterlash. Timing restrictions based on the card’s type (such as creature or sorcery) are ignored. Other restrictions are not (such as “Cast [this card] only during combat”).
1/22/2011 If you cast a card “without paying its mana cost,” you can’t pay any alternative costs. You can pay additional costs, such as kicker costs. If the card has mandatory additional costs, you must pay those.
1/22/2011 If the card has in its mana cost, you must choose 0 as its value.