The Collaborative International Dictionary of English v.0.48:

Plodder \Plod"der\, n.
   One who plods; a drudge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plodder
    n 1: someone who walks in a laborious heavy-footed manner [syn:
         trudger, plodder, slogger]
    2: someone who works slowly and monotonously for long hours
       [syn: plodder, slogger]
    3: someone who moves slowly; "in England they call a slowpoke a
       slowcoach" [syn: plodder, slowpoke, stick-in-the-mud,
       slowcoach]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "plodder":
   beast of burden, dawdle, dawdler, drone, drudge, fag, foot-dragger,
   galley slave, goldbrick, goof-off, greasy grind, grind, grub, hack,
   laggard, lie-abed, lingerer, loiterer, procrastinator, slave,
   sleepyhead, slogger, slow goer, slow-foot, slowbelly, slowpoke,
   slug, sluggard, snail, stick-in-the-mud, swot, tortoise,
   workhorse