The Collaborative International Dictionary of English v.0.48:

Merciful \Mer"ci*ful\, a. [Mercy + -ful.]
   1. Full of mercy; having or exercising mercy; disposed to
      pity and spare offenders; unwilling to punish. Opposite of
      merciless.
      [1913 Webster]

            The Lord, the Lord God, merciful and gracious. --Ex.
                                                  xxxiv. 6.
      [1913 Webster]

            Be merciful, great duke, to men of mold. --Shak.
      [1913 Webster]

   2. Unwilling to give pain; compassionate.
      [1913 Webster]

            A merciful man will be merciful to his beast. --Old
                                                  Proverb.
      [1913 Webster]

   Syn: Compassionate; tender; humane; gracious; kind; mild;
        clement; benignant.
        [1913 Webster] -- Mer"ci*ful*ly, adv. --
        Mer"ci*ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

merciful
    adj 1: showing or giving mercy; "sought merciful treatment for
           the captives"; "a merciful god" [ant: merciless,
           unmerciful]
    2: (used conventionally of royalty and high nobility) gracious;
       "our merciful king"

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "merciful":
   accepting, all-knowing, all-powerful, all-seeing, all-wise,
   almighty, beneficent, benign, benignant, big, bleeding, boundless,
   changeless, charitable, clement, commiserative, compassionate,
   condolent, considerate, creating, creative, easy, easygoing,
   eternal, eternally the same, everlasting, forbearant, forbearing,
   forgiving, generous, gentle, glorious, good, gracious, hallowed,
   highest, holy, human, humane, immortal, immutable, indulgent,
   infinite, just, kind, kindly, lax, lenient, liberal, limitless,
   loving, luminous, magnanimous, majestic, making, melting, mild,
   moderate, numinous, omnipotent, omnipresent, omniscient, one,
   patient, permanent, perpetual, pitiful, pitying, radiant, ruth,
   ruthful, sacred, shaping, soft, softhearted, sovereign, sparing,
   supreme, sympathetic, sympathizing, tender, tenderhearted,
   thoughtful, timeless, tolerant, ubiquitous, unbounded, unchanging,
   undefined, understanding, unlimited, warmhearted