Void Winnower

Your opponents can't cast spells with even mana values. (Zero is even.)
Your opponents can't block with creatures with even mana values.

Counterbalance

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.

Mana Maze

Players can't cast spells that share a color with the spell most recently cast this turn.

Talion, the Kindly Lord

Flying
As Talion enters, choose a number between 1 and 10.
Whenever an opponent casts a spell with mana value, power, or toughness equal to the chosen number, that player loses 2 life and you draw a card.

Sanctum Prelate

As this creature enters, choose a number.
Noncreature spells with mana value equal to the chosen number can't be cast.

Chalice of the Void

This artifact enters with X charge counters on it.
Whenever a player casts a spell with mana value equal to the number of charge counters on this artifact, counter that spell.

Squall, Gunblade Duelist

First strike
As Squall enters, choose a number.
Whenever one or more creatures attack one of your opponents, if any of those creatures have power or toughness equal to the chosen number, Squall deals damage equal to its power to defending player.

Extinction Event

Choose odd or even. Exile each creature with mana value of the chosen quality. (Zero is even.)

Powerbalance

Whenever an opponent casts a spell, you may reveal the top card of your library. If you do, you may cast that card without paying its mana cost if the two spells have the same mana value.