The Collaborative International Dictionary of English v.0.48:

Fruitful \Fruit"ful\, a.
   Full of fruit; producing fruit abundantly; bearing results;
   prolific; fertile; liberal; bountiful; as, a fruitful tree,
   or season, or soil; a fruitful wife. -- Fruit"ful*ly, adv.
   -- Fruit"ful*ness, n.
   [1913 Webster]

         Be fruitful and multiply and replenish the earth.
                                                  --Gen. i. 28.
   [1913 Webster]

         [Nature] By disburdening grows
         More fruitful.                           --Milton.
   [1913 Webster]

         The great fruitfulness of the poet's fancy. --Addison.

   Syn: Fertile; prolific; productive; fecund; plentiful; rich;
        abundant; plenteous. See Fertile.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fruitful
    adj 1: productive or conducive to producing in abundance; "be
           fruitful and multiply" [ant: unfruitful]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "fruitful":
   abundant, advantageous, beneficial, blooming, bounteous, bountiful,
   bursting, bursting out, copious, creative, effective, exuberant,
   fecund, fertile, flourishing, fructiferous, fructuous, generous,
   lush, luxuriant, luxurious, plenteous, plentiful, pregnant,
   productive, profitable, proliferous, prolific, rewarding, rich,
   seminal, successful, superabundant, swarming, teeming, thriving,
   uberous, useful, well-spent, worthwhile