Watcher of the Spheres

Flying
Creature spells with flying you cast cost {1} less to cast.
Whenever another creature you control with flying enters, Watcher of the Spheres gets +1/+1 until end of turn.

Starnheim Courser

Flying
Artifact and enchantment spells you cast cost {1} less to cast.

Emerald Medallion

Green spells you cast cost {1} less to cast.

Path of Ancestry

Path of Ancestry enters tapped.
{T}: Add one mana of any color in your commander's color identity. When that mana is spent to cast a creature spell that shares a creature type with your commander, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Thunderscape Familiar

First strike
Black spells and green spells you cast cost {1} less to cast.

Stormscape Familiar

Flying
White spells and black spells you cast cost {1} less to cast.

Urza's Incubator

As Urza's Incubator enters, choose a creature type.
Creature spells of the chosen type cost {2} less to cast.

City of Brass

Whenever City of Brass becomes tapped, it deals 1 damage to you.
{T}: Add one mana of any color.

Pearl Medallion

White spells you cast cost {1} less to cast.

Living Wish

You may reveal a creature or land card you own from outside the game and put it into your hand. Exile Living Wish.

Helm of Awakening

Enthusiastic Mechanaut

Flying
Artifact spells you cast cost {1} less to cast.

Mistform Warchief

Creature spells you cast that share a creature type with Mistform Warchief cost {1} less to cast.
{T}: Mistform Warchief becomes the creature type of your choice until end of turn.

Cloud Key

As Cloud Key enters, choose artifact, creature, enchantment, instant, or sorcery.
Spells you cast of the chosen type cost {1} less to cast.

Centaur Omenreader

As long as Centaur Omenreader is tapped, creature spells you cast cost {2} less to cast.

Heartstone

Activated abilities of creatures cost {1} less to activate. This effect can't reduce the mana in that cost to less than one mana.

Foundry Inspector

Artifact spells you cast cost {1} less to cast.

Sunscape Familiar

Defender (This creature can't attack.)
Green spells and blue spells you cast cost {1} less to cast.