The Collaborative International Dictionary of English v.0.48:

passerby \pas`ser*by"\, Passer-by \Pass`er-by"\, n.
   One who passes by, especially casually or by chance; one not
   directly involved in some action; a passer.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

passerby
    n 1: a person who passes by casually or by chance [syn:
         passerby, passer-by, passer]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "passerby":
   TV-viewer, adventurer, alpinist, astronaut, attestant, attestator,
   attester, beholder, bird-watcher, bystander, camper, climber,
   cojuror, comers and goers, commuter, compurgator, cosmopolite,
   cruiser, deponent, drugstore cowboy, earwitness, excursionist,
   explorer, eyewitness, fare, gaper, gazer, gazer-on, girl-watcher,
   globe-girdler, globe-trotter, goer, goggler, hajji, informant,
   informer, jet set, jet-setter, journeyer, kibitzer, looker,
   looker-on, mariner, mountaineer, observer, ogler, onlooker, palmer,
   passenger, passer, pathfinder, perceiver, percipient, pilgrim,
   pioneer, rubberneck, rubbernecker, sailor, seer,
   sidewalk superintendent, sightseer, sojourner, spectator,
   spectatress, spectatrix, straphanger, swearer, televiewer,
   television-viewer, temporary lodger, testifier, tourer, tourist,
   trailblazer, trailbreaker, transient, transient guest, traveler,
   trekker, tripper, viator, video-gazer, viewer, visiting fireman,
   voortrekker, voucher, voyager, voyageur, watcher, wayfarer,
   witness, world-traveler