The Collaborative International Dictionary of English v.0.48:

Bystander \By"stand`er\, n. [By + stander, equiv. to stander-by;
   cf. AS. big-standan to stand by or near.]
   One who stands near; a spectator; one who has no concern with
   the business transacting.
   [1913 Webster]

         He addressed the bystanders and scattered pamphlets
         among them.                              --Palfrey.
   [1913 Webster]

   Syn: Looker on; spectator; beholder; observer.
        [1913 Webster]

WordNet (r) 3.0 (2006):

bystander
    n 1: a nonparticipant spectator

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "bystander":
   TV-viewer, abutter, adjoiner, attestant, attestator, attester,
   beholder, bird-watcher, borderer, cojuror, compurgator, deponent,
   drugstore cowboy, earwitness, eyewitness, gaper, gazer, gazer-on,
   girl-watcher, goggler, immediate neighbor, informant, informer,
   kibitzer, looker, looker-on, neighbor, neighborer, observer, ogler,
   onlooker, passerby, perceiver, percipient, seer,
   sidewalk superintendent, spectator, spectatress, spectatrix,
   swearer, tangent, televiewer, television-viewer, testifier,
   video-gazer, viewer, voucher, watcher, witness