Seat of the Synod

Dryad Arbor

(Dryad Arbor isn't a spell, it's affected by summoning sickness, and it has "{T}: Add {G}.")

Myr Retriever

When Myr Retriever dies, return another target artifact card from your graveyard to your hand.

Scrap Trawler

Whenever Scrap Trawler or another artifact you control is put into a graveyard from the battlefield, return to your hand target artifact card in your graveyard with lesser mana value.

Great Furnace

Darksteel Citadel

Karn, the Great Creator

Activated abilities of artifacts your opponents control can't be activated.
+1: Until your next turn, up to one target noncreature artifact becomes an artifact creature with power and toughness each equal to its mana value.
−2: You may reveal an artifact card you own from outside the game or choose a face-up artifact card you own in exile. Put that card into your hand.

Krark-Clan Ironworks

Ancient Den

Mox Opal

Metalcraft — {T}: Add one mana of any color. Activate only if you control three or more artifacts.