The Collaborative International Dictionary of English v.0.48:

Interest \In"ter*est\, v. t. [imp. & p. p. Interested; p. pr.
   & vb. n. Interesting.] [From interess'd, p. p. of the older
   form interess, fr. F. int['e]resser, L. interesse. See
   Interest, n.]
   [1913 Webster]
   1. To engage the attention of; to awaken interest in; to
      excite emotion or passion in, in behalf of a person or
      thing; as, the subject did not interest him; to interest
      one in charitable work.
      [1913 Webster]

            To love our native country . . . to be interested in
            its concerns is natural to all men.   --Dryden.
      [1913 Webster]

            A goddess who used to interest herself in marriages.
                                                  --Addison.
      [1913 Webster]

   2. To be concerned with or engaged in; to affect; to concern;
      to excite; -- often used impersonally. [Obs.]
      [1913 Webster]

            Or rather, gracious sir,
            Create me to this glory, since my cause
            Doth interest this fair quarrel.      --Ford.
      [1913 Webster]

   3. To cause or permit to share. [Obs.]
      [1913 Webster]

            The mystical communion of all faithful men is such
            as maketh every one to be interested in those
            precious blessings which any one of them receiveth
            at God's hands.                       --Hooker.

   Syn: To concern; excite; attract; entertain; engage; occupy;
        hold.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Interested \In"ter*est*ed\ ([i^]n"t[~e]r*[e^]st*[e^]d), a. [See
   Interest, v. t.]
   [1913 Webster]
   1. Having the attention engaged; having emotion or passion
      excited; as, an interested listener.
      [1913 Webster]

   2. Having an interest; concerned in a cause or in
      consequences; liable to be affected or prejudiced; as, an
      interested witness; an interested party.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interested
    adj 1: having or showing interest; especially curiosity or
           fascination or concern; "an interested audience";
           "interested in sports"; "was interested to hear about her
           family"; "interested in knowing who was on the
           telephone"; "interested spectators" [ant: uninterested]
    2: involved in or affected by or having a claim to or share in;
       "a memorandum to those concerned"; "an enterprise in which
       three men are concerned"; "factors concerned in the rise and
       fall of epidemics"; "the interested parties met to discuss
       the business" [syn: concerned, interested]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "interested":
   a party to, absorbed, affected, agape, agog, all agog, antiblack,
   attracted, biased, burning with curiosity, cathectic, chauvinistic,
   colored, concerned, consumed with curiosity, curious,
   denominational, doctrinaire, dogmatic, drawn to, engaged,
   engrossed, enthusiastic, excited, factional, fascinated, ghoulish,
   gossipy, implicated, included, influenced, inquiring, inquisitive,
   involved, involved in, itchy, jaundiced, keen on, know-nothing,
   morbid, morbidly curious, nonobjective, one-sided, open-eyed,
   openmouthed, opinionated, overcurious, partial, partisan, party,
   passionate, piqued, predisposed, prejudiced, prejudicial,
   prepossessed, prurient, quizzical, racist, scopophiliac, sectarian,
   sectary, sectional, sexist, supercurious, superpatriotic, swayed,
   tantalized, tickled, titillated, turned-on, twisted,
   ultranationalist, undetached, undispassionate, unneutral,
   voyeuristic, warped, xenophobic