WordNet (r) 3.0 (2006):

backseat driver
    n 1: a meddler who insists on giving unwanted advice

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "backseat driver":
   Dutch uncle, Paul Pry, Polonius, Sunday driver, admonisher,
   adviser, bus driver, busman, busybody, buttinsky, cabby, cabdriver,
   chauffeur, confidant, consultant, counsel, counselor, driver,
   expert, guide, hack, hackdriver, hackman, hacky,
   hit-and-run driver, instructor, intermeddler, jitney driver,
   joyrider, kibitzer, meddler, mentor, monitor, motorist, nestor,
   orienter, preceptist, prier, pry, road hog, snoop, snooper,
   speeder, taxidriver, teacher, teamster, truck driver, trucker,
   truckman, yenta