The Collaborative International Dictionary of English v.0.48:

Freebooter \Free"boot`er\, n. [D. vrijbuiter, fr. vrijbuiten to
   plunder; vrij free + buit booty, akin to E. booty. See
   Free, and Booty, and cf. Filibuster.]
   One who plunders or pillages without the authority of
   national warfare; a member of a predatory band; a pillager; a
   buccaneer; a sea robber. --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

The Devil's Dictionary (1881-1906):

FREEBOOTER, n.  A conqueror in a small way of business, whose
annexations lack of the sanctifying merit of magnitude.