Stronghold Machinist

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

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.

Waterfront Bouncer

{U}, {T}, Discard a card: Return target creature to its owner's hand.

Diplomatic Escort

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

Alexi, Zephyr Mage

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

Bog Witch

Instigator

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

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.

Tortured Existence

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

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.

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.)

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.

Chamber of Manipulation

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

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.

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.)

Latulla, Keldon Overseer

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

Escape Artist

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

Undertaker

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

Null Brooch

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

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.)