The Collaborative International Dictionary of English v.0.48:

Windfall \Wind"fall`\, n.
   1. Anything blown down or off by the wind, as fruit from a
      tree, or the tree itself, or a portion of a forest
      prostrated by a violent wind, etc. "They became a windfall
      upon the sudden." --Bacon.
      [1913 Webster]

   2. An unexpected legacy, or other gain.
      [1913 Webster]

            He had a mighty windfall out of doubt. --B. Jonson.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

windfall
    n 1: fruit that has fallen from the tree
    2: 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:

29 Moby Thesaurus words for "windfall":
   boast, bonus, buried treasure, catch, diamond, discovery, find,
   finding, foundling, gem, godsend, good thing, gravy, jewel, pearl,
   plum, pride, pride and joy, prize, treasure, treasure trove,
   trophy, trouvaille, trove, waifs, waifs and strays, windfall money,
   windfall profit, winner