The Collaborative International Dictionary of English v.0.48:

Congenial \Con*gen"ial\ (?; 106), a. [Pref. con- + genial.]
   1. Partaking of the same nature; allied by natural
      characteristics; kindred; sympathetic.
      [1913 Webster]

            Congenial souls! whose life one avarice joins.
                                                  --Pope.
      [1913 Webster]

            two congenial spirits united . . . by mutual
            confidence and reciprocal virtues     --T. L.
                                                  Peacock

   2. Naturally adapted; suited to the disposition; as, a
      congenial atmosphere to work in. "Congenial clime." --C.
      J. Fox.
      [1913 Webster]

            To defame the excellence with which it has no
            sympathy . . . is its congenial work. --I. Taylor.
      [1913 Webster]

   3. (Bot.) capable of cross-fertilization or of being grafted;
      -- used of plants.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

congenial
    adj 1: suitable to your needs; "a congenial atmosphere to work
           in"; "two congenial spirits united...by mutual confidence
           and reciprocal virtues"- T.L.Peacock [ant:
           incompatible, uncongenial]
    2: (used of plants) capable of cross-fertilization or of being
       grafted

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "congenial":
   accordant, affable, affinitive, affirmative, agnate, agreeable,
   agreeing, akin, allied, amiable, amicable, answerable,
   appertaining, associative, at one, attuned, blissful, brotherly,
   cheerful, civil, clubbable, clubbish, clubby, coexistent,
   coexisting, cognate, coherent, coincident, coinciding,
   commensurate, communicative, companionable, companionate,
   comparable, comparative, compatible, complaisant, concordant,
   concurring, conformable, congeneric, congenerous, congruent,
   congruous, connate, connatural, connected, connective,
   consentaneous, consentient, consistent, consonant, conspecific,
   cooperating, cooperative, cordial, correlative, correspondent,
   corresponding, courteous, desirable, dulcet, empathetic, empathic,
   en rapport, enate, enjoyable, equivalent, fair, fair and pleasant,
   favorable, felicific, felicitous, fine, fit for society,
   fond of society, fraternal, frictionless, friendlike, friendly,
   genial, good, goodly, gracious, grateful, gratifying, gregarious,
   harmonious, heart-warming, honeyed, hospitable, in accord,
   in agreement, in concert, in rapport, in sync, in synchronization,
   in tune, inaccordance, inharmony, kind, likable, like-minded,
   linking, mellifluous, mellow, neighborlike, neighborly, nice,
   of a piece, of like mind, of one mind, on all fours, peaceable,
   peaceful, pertaining, pertinent, pleasant, pleasing, pleasurable,
   pleasure-giving, pleasureful, positive, proportionable,
   proportional, proportionate, reconcilable, referable, referring,
   relating, relational, relative, responsibilities, rewarding,
   satisfying, self-consistent, simpatico, sisterly, sociable, social,
   social-minded, sweet, symbiotic, sympathetic, sympathique,
   synchronized, synchronous, together, unanimous, understanding,
   unhostile, uniform, unisonant, unisonous, united, urbane, welcome,
   well-affected, well-disposed, well-intentioned, well-meaning,
   well-meant