Stronghold Biologist

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

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.

Greenseeker

{G}, {T}, Discard a card: Search your library for a basic land card, reveal it, put it into your hand, then shuffle.

Dreamscape Artist

{2}{U}, {T}, Discard a card, Sacrifice a land: Search your library for up to two basic land cards, put them onto the battlefield, then shuffle.

Alexi, Zephyr Mage

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

Trickster Mage

{U}, {T}, Discard a card: You may tap or untap target artifact, creature, or land.

Rhystic Study

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

Cloudseeder

Flying
{U}, {T}, Discard a card: Create a 1/1 blue Faerie creature token named Cloud Sprite. It has flying and "Cloud Sprite can block only creatures with flying."

Llanowar Mentor

{G}, {T}, Discard a card: Create a 1/1 green Elf Druid creature token named Llanowar Elves. It has "{T}: Add {G}."

Island

Last March of the Ents

This spell can't be countered.
Draw cards equal to the greatest toughness among creatures you control, then put any number of creature cards from your hand onto the battlefield.

Stampede Driver

{1}{G}, {T}, Discard a card: Creatures you control get +1/+1 and gain trample until end of turn.

Ancient Silver Dragon

Flying
Whenever Ancient Silver Dragon deals combat damage to a player, roll a d20. Draw cards equal to the result. You have no maximum hand size for the rest of the game.

Balloon Peddler

{U}, {T}, Discard a card: Target creature gains flying until end of turn.

Tolarian Sentinel

Flying
{U}, {T}, Discard a card: Return target permanent you control to its owner's hand.

Thought Vessel

You have no maximum hand size.
{T}: Add {C}.

Alhammarret's Archive

If you would gain life, you gain twice that much life instead.
If you would draw a card except the first one you draw in each of your draw steps, draw two cards instead.

Sophic Centaur

{2}{G}{G}, {T}, Discard a card: You gain 2 life for each card in your hand.

Silverglade Pathfinder

{1}{G}, {T}, Discard a card: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Diplomatic Escort

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

Sea Gate, Reborn

As Sea Gate, Reborn enters the battlefield, you may pay 3 life. If you don't, it enters the battlefield tapped.
{T}: Add {U}.

Sea Gate Restoration

Draw cards equal to the number of cards in your hand plus one. You have no maximum hand size for the rest of the game.

Sea Gate Restoration // Sea Gate, Reborn

Journey to the Oracle

You may put any number of land cards from your hand onto the battlefield. Then if you control eight or more lands, you may discard a card. If you do, return Journey to the Oracle to its owner's hand.

Jadzi, Oracle of Arcavios

Discard a card: Return Jadzi, Oracle of Arcavios to its owner's hand.
Magecraft — Whenever you cast or copy an instant or sorcery spell, reveal the top card of your library. If it's a nonland card, you may cast it by paying {1} rather than paying its mana cost. If it's a land card, put it onto the battlefield.

Jadzi, Oracle of Arcavios // Journey to the Oracle

Forest

Waterfront Bouncer

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

Stronghold Machinist

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

Jolrael, Voice of Zhalfir

At the beginning of combat on your turn, up to one target land you control becomes an X/X green and blue Bird creature with flying and haste until end of turn, where X is the number of cards in your hand. It's still a land.
Whenever a land creature you control deals combat damage to a player, draw a card.

Dawnstrider

{G}, {T}, Discard a card: Prevent all combat damage that would be dealt this turn.