WordNet (r) 3.0 (2006):

last straw
    n 1: the final irritation that stretches your patience beyond
         the limit

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "last straw":
   causa causans, causa finalis, causal sequence, causing cause,
   efficient cause, final cause, first cause, formal cause,
   immanent cause, immediate cause, occasional cause, provocation,
   proximate cause, remote cause, stand, the last straw, the limit,
   transient cause, ultimate cause