Circular Logic

Counter target spell unless its controller pays {1} for each card in your graveyard.
Madness {U} (If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)

Gastal Thrillroller

Trample, haste
When this Vehicle enters, it becomes an artifact creature until end of turn.
Crew 2
{2}{R}, Discard a card: Return this card from your graveyard to the battlefield with a finality counter on it. Activate only as a sorcery.

Null Brooch

{2}, {T}, Discard your hand: Counter target noncreature spell.

Chamber of Manipulation

Enchant land
Enchanted land has "{T}, Discard a card: Gain control of target creature until end of turn."

Latulla, Keldon Overseer

{X}{R}, {T}, Discard two cards: Latulla deals X damage to any target.

Diplomatic Escort

{U}, {T}, Discard a card: Counter target spell or ability that targets a creature.

Quasiduplicate

Create a token that's a copy of target creature you control.
Jump-start (You may cast this card from your graveyard by discarding a card in addition to paying its other costs. Then exile this card.)

Escape Artist

This creature can't be blocked.
{U}, Discard a card: Return this creature to its owner's hand.

Alexi, Zephyr Mage

{X}{U}, {T}, Discard two cards: Return X target creatures to their owners' hands.

Putrid Imp

Discard a card: This creature gains flying until end of turn.
Threshold — As long as seven or more cards are in your graveyard, this creature gets +1/+1 and can't block.

Overtaker

{3}{U}, {T}, Discard a card: Untap target creature and gain control of it until end of turn. That creature gains haste until end of turn.

Divining Witch

{1}{B}, {T}, Discard a card: Choose a card name. Exile the top six cards of your library, then reveal cards from the top of your library until you reveal a card with the chosen name. Put that card into your hand and exile all other cards revealed this way.

Bog Witch

Instigator

{1}{B}{B}, {T}, Discard a card: Creatures target player controls attack this turn if able.

Undertaker

{B}, {T}, Discard a card: Return target creature card from your graveyard to your hand.

Tortured Existence

{B}, Discard a creature card: Return target creature card from your graveyard to your hand.

Solphim, Mayhem Dominus

If a source you control would deal noncombat damage to an opponent or a permanent an opponent controls, it deals double that damage to that player or permanent instead.
{1}{R/P}{R/P}, Discard two cards: Put an indestructible counter on Solphim. ({R/P} can be paid with either {R} or 2 life.)

Stronghold Biologist

{U}{U}, {T}, Discard a card: Counter target creature spell.

Kozilek, the Great Distortion

When you cast this spell, if you have fewer than seven cards in hand, draw cards equal to the difference.
Menace
Discard a card with mana value X: Counter target spell with mana value X.