Urza, Lord High Artificer (DMR #296)

When Urza enters, create a 0/0 colorless Construct artifact creature token with “This token gets +1/+1 for each artifact you control.”

Tap an untapped artifact you control: Add {U}.

{5}: Shuffle your library, then exile the top card. Until end of turn, you may play that card without paying its mana cost.