WordNet (r) 3.0 (2006):

kibitzer
    n 1: (Yiddish) a meddler who offers unwanted advice to others

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "kibitzer":
   Dutch uncle, Paul Pry, Polonius, TV-viewer, admonisher, adviser,
   backseat driver, beholder, bird-watcher, busybody, butt-in,
   buttinsky, bystander, confidant, consultant, counsel, counselor,
   drugstore cowboy, expert, eyewitness, gaper, gazer, gazer-on,
   girl-watcher, goggler, guide, instructor, intermeddler, looker,
   looker-on, meddler, mentor, monitor, nestor, observer, ogler,
   onlooker, orienter, perceiver, percipient, pragmatist, preceptist,
   prier, pry, quidnunc, rubberneck, seer, sidewalk superintendent,
   snoop, snooper, spectator, spectatress, spectatrix, teacher,
   televiewer, television-viewer, video-gazer, viewer, watcher,
   witness, yenta