The Collaborative International Dictionary of English v.0.48:

hardworking \hard"work`ing\ adj.
   1. habitually working diligently and for long hours.

   Syn: industrious, tireless, untiring.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hardworking
    adj 1: characterized by hard work and perseverance [syn:
           hardworking, industrious, tireless, untiring]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "hardworking":
   ardent, assiduous, diligent, drudging, energetic, fervent,
   grinding, grubbing, hard, indefatigable, industrious, laboring,
   laborious, never idle, pegging, plodding, plugging, relentless,
   sedulous, slaving, sleepless, slogging, straining, strenuous,
   striving, struggling, sweating, tireless, toiling, unflagging,
   unremitting, unsleeping, unsparing, unwearied, vehement, working,
   zealous