Esper Sentinel

Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays {X}, where X is Esper Sentinel's power.

Blind Obedience

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Artifacts and creatures your opponents control enter tapped.

Aether Barrier

Whenever a player casts a creature spell, that player sacrifices a permanent of their choice unless they pay {1}.

In the Eye of Chaos

Whenever a player casts an instant spell, counter it unless that player pays {X}, where X is its mana value.

Crawlspace

No more than two creatures can attack you each combat.

Mana Chains

Enchant creature
Enchanted creature has "Cumulative upkeep {1}." (At the beginning of its controller's upkeep, that player puts an age counter on it, then sacrifices it unless they pay its upkeep cost for each age counter on it.)

Stasis

Players skip their untap steps.
At the beginning of your upkeep, sacrifice Stasis unless you pay {U}.

Slow Motion

Enchant creature
At the beginning of the upkeep of enchanted creature's controller, that player sacrifices that creature unless they pay {2}.
When Slow Motion is put into a graveyard from the battlefield, return Slow Motion to its owner's hand.

Ice Cave

Whenever a player casts a spell, any other player may pay that spell's mana cost. If a player does, counter the spell. (Mana cost includes color.)

Rhystic Circle

{1}: Any player may pay {1}. If no one does, the next time a source of your choice would deal damage to you this turn, prevent that damage.

Smothering Tithe

Whenever an opponent draws a card, that player may pay {2}. If the player doesn't, you create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Grand Arbiter Augustin IV

White spells you cast cost {1} less to cast.
Blue spells you cast cost {1} less to cast.
Spells your opponents cast cost {1} more to cast.

Ghostly Prison

Creatures can't attack you unless their controller pays {2} for each creature they control that's attacking you.

Tariff

Each player sacrifices the creature they control with the highest mana value unless they pay that creature's mana cost. If two or more creatures a player controls are tied for highest, that player chooses one.

Soul Barrier

Whenever an opponent casts a creature spell, Soul Barrier deals 2 damage to that player unless they pay {2}.

Propaganda

Equilibrium

Whenever you cast a creature spell, you may pay {1}. If you do, return target creature to its owner's hand.

Rhystic Study

Whenever an opponent casts a spell, you may draw a card unless that player pays {1}.

Jin-Gitaxias, Progress Tyrant

Whenever you cast an artifact, instant, or sorcery spell, copy that spell. You may choose new targets for the copy. This ability triggers only once each turn. (A copy of a permanent spell becomes a token.)
Whenever an opponent casts an artifact, instant, or sorcery spell, counter that spell. This ability triggers only once each turn.

Spelltithe Enforcer

Whenever an opponent casts a spell, that player sacrifices a permanent of their choice unless they pay {1}.