May/Can I have your attention, please? – used to attract the attention of a group of people who are talking among themselves to make an announcement, etc.
Attention, please. – used to introduce official announcements, especially over a loudspeaker.
I say. – sometimes used in conversation to draw particular attention to the sentence which follows. Addressed more often to groups of people than individuals.
Look here! – used to attract someone’s attention to something of which you disapprove. Often indignant.
Just a minute. – used to someone who is going away before you think he should (you still have something to tell him) or to stop someone in the middle of what he is saying in order to voice an objection.
Hey! – used wither to express indignation or to warn someone quickly of some danger. If used simply to attract attention it is rude, or at least familiar.
Look/ watch out! – to warn someone of danger.
Mind …! – meaning “be careful (of)”