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)