The Collaborative International Dictionary of English v.0.48:

Complementary \Com`ple*men"ta*ry\, a.
   Serving to fill out or to complete; as, complementary
   numbers.
   [1913 Webster]

   Complementary colors. See under Color.

   Complementary angles (Math.), two angles whose sum is
      90[deg].
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Complementary \Com`ple*men"ta*ry\, n. [See Complimentary.]
   One skilled in compliments. [Obs.] --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

complementary
    adj 1: of words or propositions so related that each is the
           negation of the other; "`male' and `female' are
           complementary terms"
    2: acting as or providing a complement (something that completes
       the whole) [syn: complemental, complementary,
       completing]
    n 1: either one of two chromatic colors that when mixed together
         give white (in the case of lights) or grey (in the case of
         pigments); "yellow and blue are complementaries" [syn:
         complementary color, complementary]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "complementary":
   analogous, answering, coequal, complemental, completing,
   completive, completory, consummative, consummatory, correspondent,
   corresponding, culminative, equipollent, equivalent, filling,
   fulfilling, homologous, perfective, reciprocal, reciprocative,
   tantamount