The Collaborative International Dictionary of English v.0.48:

Plunderer \Plun"der*er\, n.
   One who plunders or pillages.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plunderer
    n 1: someone who takes spoils or plunder (as in war) [syn:
         plunderer, pillager, looter, spoiler, despoiler,
         raider, freebooter]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "plunderer":
   depredator, despoiler, forager, forayer, freebooter, looter,
   marauder, pillager, raider, rapparee, ravager, ravisher, reiver,
   rifler, sacker, spoiler, spoliator, wrecker