At the beginning of each player's draw step, if Howling Mine is untapped, that player draws an additional card.
10/4/2004 If Howling Mine leaves the battlefield before it resolves, then the last known tap or untap state of the card is used for resolution.
10/4/2004 It does not trigger at all if this is tapped at the start of the draw step, and it checks this again on resolution.
10/4/2004 The additional draw is separate from any other draw during your draw step. It happens when the triggered ability resolves.
4/15/2013 The triggered ability is put onto the stack after you have already drawn your card for the turn.