WordNet (r) 3.0 (2006):

simmer down
    v 1: become quiet or calm, especially after a state of
         agitation; "After the fight both men need to cool off.";
         "It took a while after the baby was born for things to
         settle down again." [syn: calm, calm down, cool off,
         chill out, simmer down, settle down, cool it]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "simmer down":
   calm down, collect, collect oneself, compose oneself, control,
   cool, cool it, cool off, get organized, quiet down, rein, relax,
   repress, restrain, smother, sober down, subside, suppress,
   take it easy, unwind