The Collaborative International Dictionary of English v.0.48:

Spare \Spare\, a. [Compar. Sparer; superl. Sparest; -- not
   used in all the senses of the word.] [AS. spaer sparing. Cf.
   Spare, v. t. ]
   1. Scanty; not abundant or plentiful; as, a spare diet.
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   2. Sparing; frugal; parsimonious; chary.
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            He was spare, but discreet of speech. --Carew.
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   3. Being over and above what is necessary, or what must be
      used or reserved; not wanted, or not used; superfluous;
      as, I have no spare time.
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            If that no spare clothes he had to give. --Spenser.
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   4. Held in reserve, to be used in an emergency; as, a spare
      anchor; a spare bed or room.
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   5. Lean; wanting flesh; meager; thin; gaunt.
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            O, give me the spare men, and spare me the great
            ones.                                 --Shak.
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   6. Slow. [Obs. or prov. Eng.] --Grose.
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The Collaborative International Dictionary of English v.0.48:

Sparer \Spar"er\, n.
   One who spares.
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WordNet (r) 3.0 (2006):

sparer
    n 1: someone who refrains from injuring or destroying