The Collaborative International Dictionary of English v.0.48:

Looker \Look"er\, n.
   1. One who looks.
      [1913 Webster]

   2. A person who is physically very attractive, especially a
      beautiful woman.
      [PJC]

   Looker-on, a spectator; an onlooker; one that looks on, but
      has no agency or part in an affair.
      [1913 Webster]

            Did not this fatal war affront thy coast,
            Yet sattest thou an idle looker-on ?  --Fairfax.
      [1913 Webster]

WordNet (r) 3.0 (2006):

looker
    n 1: a close observer; someone who looks at something (such as
         an exhibition of some kind); "the spectators applauded the
         performance"; "television viewers"; "sky watchers
         discovered a new star" [syn: spectator, witness,
         viewer, watcher, looker]
    2: a very attractive or seductive looking woman [syn: smasher,
       stunner, knockout, beauty, ravisher, sweetheart,
       peach, lulu, looker, mantrap, dish]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "looker":
   Miss America, TV-viewer, accepter, acquirer, addressee, audience,
   auditor, bathing beauty, beau ideal, beaut, beauty,
   beauty contest winner, beauty queen, beholder, belle, bird-watcher,
   bunny, bystander, charmer, consignee, cover girl, dazzler, dream,
   drugstore cowboy, enchantress, eyeful, eyewitness, gaper, gazer,
   gazer-on, getter, girl-watcher, goggler, great beauty, hearer,
   holder, kibitzer, knockout, lady fair, listener, looker-on, lovely,
   model, observer, obtainer, ogler, onlooker, paragon, payee, peach,
   perceiver, percipient, pinup, pinup girl, procurer, pussycat,
   raving beauty, receiver, recipient, reigning beauty, seer,
   sex kitten, sidewalk superintendent, slick chick, spectator,
   spectatress, spectatrix, stunner, taker, televiewer,
   television-viewer, trustee, video-gazer, viewer, watcher,
   witness