WordNet (r) 3.0 (2006):

buttinsky
    n 1: a meddler who tends to butt in

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "buttinsky":
   Dutch uncle, Paul, Polonius, admonisher, adviser, backseat driver,
   butt-in, confidant, consultant, counsel, counselor, crasher,
   encroacher, expert, gate-crasher, guide, infiltrator, instructor,
   interloper, intermeddler, intruder, invader, kibitzer, meddler,
   mentor, monitor, nestor, nose, orienter, pragmatist, preceptist,
   prier, quidnunc, rubberneck, snoop, teacher, trespasser,
   unwelcome guest