Counterspell

Naiad of Hidden Coves

During turns other than yours, spells you cast cost {1} less to cast.

Island

Mana Leak

Counter target spell unless its controller pays {3}.

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Didn't Say Please

Counter target spell. Its controller mills three cards.

Disallow

Counter target spell, activated ability, or triggered ability. (Mana abilities can't be targeted.)

Behold the Multiverse

Scry 2, then draw two cards.
Foretell {1}{U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Logic Knot

Delve (Each card you exile from your graveyard while casting this spell pays for {1}.)
Counter target spell unless its controller pays {X}.