Heartstone
Artifact (CMC 3)
Activated abilities of creatures cost
less to activate. This effect can't reduce the mana in that cost to less than one mana.
Decks with Heartstone
[PLIST] The List #1333 (U)
[STH] Stronghold #134 (U)
[H09] Premium Deck Series: Slivers #26 (U)