The Collaborative International Dictionary of English v.0.48:

On-looker \On"-look`er\, n.
   A person who observes an event; a looker-on; a spectator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

onlooker
    n 1: someone who looks on [syn: onlooker, looker-on]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "onlooker":
   TV-viewer, abutter, adjoiner, beholder, bird-watcher, borderer,
   bystander, drugstore cowboy, eyewitness, gaper, gazer, gazer-on,
   girl-watcher, goggler, immediate neighbor, kibitzer, looker,
   looker-on, neighbor, neighborer, observer, ogler, perceiver,
   percipient, seer, sidewalk superintendent, spectator, spectatress,
   spectatrix, tangent, televiewer, television-viewer, video-gazer,
   viewer, watcher, witness