Counterbalance is a uncommon Enchantment card from Mystery Booster 2 (MB2), collector number 157. Current market price: $2.51 (foil $24.14), TCGplayer market prices via Scryfall.
Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, counter that spell if it has the same mana value as the revealed card.