The Collaborative International Dictionary of English v.0.48:

Sponger \Spon"ger\ (-j[~e]r), n.
   1. One who sponges, or uses a sponge.
      [1913 Webster]

   2. One employed in gathering sponges.
      [1913 Webster]

   3. Fig.: A parasitical dependent; a hanger-on.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sponger
    n 1: a workman employed to collect sponges
    2: a follower who hangs around a host (without benefit to the
       host) in hope of gain or advantage [syn: leech, parasite,
       sponge, sponger]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "sponger":
   barnacle, beat, beggar, bloodsucker, bummer, cadger,
   coupon clippers, deadbeat, drone, freeloader, hanger-on, idle rich,
   leech, leisure class, lounge lizard, lumpen proletariat, mendicant,
   moocher, nonworker, panhandler, parasite, rentiers, smell-feast,
   spiv, sponge, sucker, the unemployable, the unemployed