The Collaborative International Dictionary of English v.0.48:

Donor \Do"nor\, n. [F. donneur, OF. daneor, fr. donner. See
   Donee, and cf. Donator.]
   1. One who gives or bestows; one who confers anything
      gratuitously; a benefactor. Inverse of recipient.
      [1913 Webster]

   2. (Law) One who grants an estate; in later use, one who
      confers a power; -- the opposite of donee. --Kent.
      [1913 Webster]

            Touching, the parties unto deeds and charters, we
            are to consider as well the donors and granters as
            the donees or grantees.               --Spelman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

donor
    n 1: person who makes a gift of property [syn: donor, giver,
         presenter, bestower, conferrer]
    2: (medicine) someone who gives blood or tissue or an organ to
       be used in another person (the host)

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "donor":
   Maecenas, Santa Claus, almoner, almsgiver, angel, assignor,
   awarder, backer, benefactor, benefactress, bestower, caterer,
   chandler, cheerful giver, commissariat, commissary, conferrer,
   consignor, contributor, donator, fairy godmother, feoffor,
   financer, funder, furnisher, giver, grantor, imparter,
   lady bountiful, manciple, merchant, patron, patroness,
   philanthropist, presenter, provider, provisioner, purveyor,
   quartermaster, retailer, settler, steward, stock clerk,
   storekeeper, subscriber, sugar daddy, supplier, supporter, sutler,
   testate, testator, testatrix, victualer, vivandier, vouchsafer



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DONOR. He who makes a gift. (q.v.)