The Collaborative International Dictionary of English v.0.48:

Godsend \God"send`\ (g[o^]d"s[e^]nd), n.
   Something sent by God; an unexpected acquisiton or piece of
   good fortune.
   [1913 Webster]

WordNet (r) 3.0 (2006):

godsend
    n 1: a sudden happening that brings good fortune (as a sudden
         opportunity to make money); "the demand for testing has
         created a boom for those unregulated laboratories where
         boxes of specimen jars are processed like an assembly line"
         [syn: boom, bonanza, gold rush, gravy, godsend,
         manna from heaven, windfall, bunce]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "godsend":
   advantage, benediction, benefit, blessing, boast, bonanza, boon,
   catch, diamond, find, gem, gift, good thing, jewel,
   loaves and fishes, manna, manna from heaven, pearl, plum, pride,
   pride and joy, prize, treasure, trophy, trouvaille, windfall,
   winner