WordNet (r) 3.0 (2006):

backslapper
    n 1: someone who demonstrates enthusiastic or excessive
         cordiality

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "backslapper":
   adulator, apple-polisher, ass-licker, backscratcher, blarneyer,
   bootlick, bootlicker, brown-nose, brownie, cajoler, clawback,
   courtier, creature, cringer, dupe, fawner, flatterer, flunky,
   footlicker, groveler, handshaker, helot, instrument, jackal,
   kowtower, lackey, led captain, lickspit, lickspittle, mealymouth,
   minion, peon, puppet, serf, slave, spaniel, stooge, suck,
   sycophant, timeserver, toad, toady, tool, truckler, tufthunter,
   wheedler, yes-man