Free mechanic

Free spells are spells which when played also untap an amount of lands that would normally be used to play the spell. Free spells, all of which are blue, were originally all printed in Urza block. The mechanic made a cameo years later in Dominaria, and Rivals of Ixalan has Zacama, Primal Calamity, which has the safety valve of requiring casting. This mechanic proved to be fundamentally broken.

Rulings

• You don't choose which lands to untap until the instant spell, sorcery spell, or triggered ability resolves.
• You may untap any lands. This includes lands you didn't tap to produce mana used to cast the spell, lands you don't control, and lands that have shroud or otherwise can't be targeted by spells or abilities.
• If a "free" instant or sorcery spell is countered (either by another spell or ability like Cancel or because all of its targets have become illegal), none of its effects happen. You won't get to untap any lands.
• If a "free" permanent enters the battlefield under your control as the result of a spell or ability, its "enters the battlefield" ability will still trigger even though it wasn't cast. You'll get to untap lands when the ability resolves.

From Urza's Saga

• Great Whale — 5/5 Whale creature.
• Peregrine Drake — 2/3 Drake creature with flying.
• Rewind — an instant with "counter target spell."
• Time Spiral — a spell that allows you to shuffle your hand plus graveyard into your library and to draw seven cards, but is removed from the game afterwards.

From Urza's Legacy

• Cloud of Faeries — a 1/1 Faerie creature with flying.
• Frantic Search — an instant with "Draw two cards, then choose and discard two cards."
• Palinchron — 4/5 Illusion creature with flying and ": Return Palinchron to owner's hand."
• Snap — an instant with "Return target creature to owner's hand."

From Urza's Destiny

• Treachery — an Enchant Creature - Aura with "You control enchanted creature."

From Dominaria

• Unwind — an instant with "Counter target noncreature spell."