depending on why and mood, I either offer them a hug to calm them down or put them somewhere safe to get over it!
sometimes it is not tantrum anymore, but more fear that they feel that they can;t calm down IYGWIM....and sometimes it is pure rage!!
either way, still do not give in as each time will be longer and harder as they will always know that they can win......
for my son, who is now 5, if it has been ofset by anyone other than me, me hugging him will calm him down and he then often dozes off.
if I the reason or cause, then it is always better to leave him for a while where he cannot see me,.....he then calms down quicker.... if he can see me it keeps reigniting his anger.