The Collaborative International Dictionary of English v.0.48:

Boon \Boon\ (b[=oo]n), n. [OE. bone, boin, a petition, fr. Icel.
   b[=o]n; akin to Sw. & Dan. b[aum]n, AS. b[=e]n, and perh. to
   E. ban; but influenced by F. bon good, fr. L. bonus.
   [root]86. See 2d Ban, Bounty.]
   1. A prayer or petition. [Obs.]
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            For which to God he made so many an idle boon.
                                                  --Spenser.
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   2. That which is asked or granted as a benefit or favor; a
      gift; a benefaction; a grant; a present.
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            Every good gift and every perfect boon is from
            above.                                --James i. 17
                                                  (Rev. Ver. ).
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The Collaborative International Dictionary of English v.0.48:

Boon \Boon\, a. [F. bon. See Boon, n.]
   1. Good; prosperous; as, boon voyage. [Obs.]
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   2. Kind; bountiful; benign.
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            Which . . . Nature boon
            Poured forth profuse on hill, and dale, and plain.
                                                  --Milton.
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   3. Gay; merry; jovial; convivial.
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            A boon companion, loving his bottle.  --Arbuthnot.
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The Collaborative International Dictionary of English v.0.48:

Boon \Boon\, n. [Scot. boon, bune, been, Gael. & Ir. bunach
   coarse tow, fr. bun root, stubble.]
   The woody portion flax, which is separated from the fiber as
   refuse matter by retting, braking, and scutching.
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WordNet (r) 3.0 (2006):

boon
    adj 1: very close and convivial; "boon companions"
    n 1: a desirable state; "enjoy the blessings of peace"; "a
         spanking breeze is a boon to sailors" [syn: blessing,
         boon]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "boon":
   advantage, avail, award, behalf, behoof, benediction, benefaction,
   benefit, benevolence, benison, blessing, blithe, blithesome,
   compliment, convivial, favor, festive, free and easy, gain, gay,
   gift, gleeful, godsend, good, grace, gratuity,
   hail-fellow-well-met, hearty, interest, jocund, jolly, jovial,
   largess, loaves and fishes, manna, manna from heaven, mirthful,
   present, profit, reward, welfare, well-being, world of good