The Collaborative International Dictionary of English v.0.48:

Zealous \Zeal"ous\ (?; 277), a. [LL. zelosus. See Zeal.]
   [1913 Webster]
   1. Filled with, or characterized by, zeal; warmly engaged, or
      ardent, in behalf of an object.
      [1913 Webster]

            He may be zealous in the salvation of souls. --Law.
      [1913 Webster]

   2. Filled with religious zeal. [Obs.] --Shak.
      [1913 Webster] -- Zeal"ous*ly, adv. -- Zeal"ous*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zealous
    adj 1: marked by active interest and enthusiasm; "an avid sports
           fan" [syn: avid, zealous]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "zealous":
   abandoned, ablaze, acquiescent, afire, agitated, agreeable,
   alacritous, all-overish, amenable, anxious, anxioused up,
   apprehensive, ardent, assiduous, avid, boiling over, bothered,
   breathless, buggy, bugs, burning, committed, compliant, concerned,
   consenting, content, cooperative, cordial, dedicated, delirious,
   devoted, devout, diligent, disposed, disquieted, disturbed, docile,
   drunk, eager, earnest, energetic, enthusiastic, excited, exuberant,
   fain, faithful, fanatic, fanatical, favorable, favorably disposed,
   favorably inclined, fearful, febrile, fervent, fervid, fevered,
   feverish, fiery, fired, flaming, flushed, foreboding, forward,
   frenetic, game, glowing, gung ho, hard, hard-core, hardworking,
   hearty, heated, hot, hot-blooded, hotheaded, impassioned,
   in a pucker, in a stew, in earnest, in the mind, in the mood,
   inclined, indefatigable, industrious, infatuated, inflamed,
   intense, intent, intent on, intoxicated, keen, laborious, lively,
   loyal, madcap, minded, misgiving, nervous, never idle, nutty,
   obsessed, on fire, on tenterhooks, overanxious, overapprehensive,
   overdevout, overreligious, overrighteous, overzealous, passionate,
   perfervid, perturbed, pliant, possessed, predisposed, prompt,
   prone, quick, rabid, ready, ready and willing, receptive, red-hot,
   relentless, resolute, responsive, sanctimonious, scorching,
   sedulous, serious, sincere, sleepless, solicitous, spirited,
   steaming, steamy, strained, strenuous, suspenseful, tense,
   tireless, totally committed, tractable, troubled, ultrareligious,
   uneasy, unflagging, unremitting, unrestrained, unsleeping,
   unsparing, unwearied, vehement, vigorous, warm, well-disposed,
   well-inclined, white-hot, wild-eyed, willed, willing,
   willinghearted, zealotic