The List

Knowledge Pool

Imprint — When this artifact enters, each player exiles the top three cards of their library.
Whenever a player casts a spell from their hand, that player exiles it. If the player does, they may cast a spell from among other cards exiled with this artifact without paying its mana cost.

Mirrodin Besieged

Imprint — When Knowledge Pool enters the battlefield, each player exiles the top three cards of his or her library.
Whenever a player casts a spell from his or her hand, that player exiles it. If the player does, he or she may cast another nonland card exiled with Knowledge Pool without paying that card's mana cost.