The Collaborative International Dictionary of English v.0.48:

Observer \Ob*serv"er\, n.
   1. One who observes, or pays attention to, anything;
      especially, one engaged in, or trained to habits of, close
      and exact observation; as, an astronomical observer.
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            The observed of all observers.        --Shak.
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            Careful observers may foretell the hour,
            By sure prognostic, when to dread a shower. --Swift.
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   2. One who keeps any law, custom, regulation, rite, etc.; one
      who conforms to anything in practice. "Diligent observers
      of old customs." --Spenser.
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            These . . . hearkened unto observers of times.
                                                  --Deut. xviii.
                                                  14.
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   3. One who fulfills or performs; as, an observer of his
      promises.
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   4. A sycophantic follower. [Obs.] --Beau. & Fl.
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WordNet (r) 3.0 (2006):

observer
    n 1: a person who becomes aware (of things or events) through
         the senses [syn: perceiver, percipient, observer,
         beholder]
    2: an expert who observes and comments on something [syn:
       observer, commentator]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "observer":
   TV-viewer, ace, air force, beholder, bird-watcher, bomber pilot,
   bystander, combat pilot, drugstore cowboy, examinant, examiner,
   eyewitness, fighter pilot, flyboy, gaper, gazer, gazer-on,
   girl-watcher, goggler, inspector, investigator, kibitzer, looker,
   looker-on, military pilot, naval pilot, ogler, onlooker, perceiver,
   percipient, scrutineer, seer, sidewalk superintendent, spectator,
   spectatress, spectatrix, suicide pilot, televiewer,
   television-viewer, tester, video-gazer, viewer, visitator, visitor,
   watcher, witness