The Collaborative International Dictionary of English v.0.48:

Herculean \Her*cu"le*an\, a. [L. herculeus, fr. Hercules: cf. F.
   hercul['e]en. See Hercules.]
   [1913 Webster]
   1. Requiring the strength of Hercules; hence, very great,
      difficult, or dangerous; as, an Herculean task.
      [1913 Webster]

   2. Having extraordinary strength or size; as, Herculean
      limbs. "Herculean Samson." --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

herculean
    adj 1: displaying superhuman strength or power; "herculean
           exertions" [syn: herculean, powerful]
    2: extremely difficult; requiring the strength of a Hercules; "a
       herculean task"

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "Herculean":
   Antaean, Atlantean, Briarean, Brobdingnagian, Cyclopean,
   Gargantuan, Homeric, abstruse, abysmal, arduous, astronomic,
   backbreaking, brutal, burdensome, colossal, complex, critical,
   crushing, delicate, demanding, difficile, difficult, effortful,
   elephantine, enormous, epic, exacting, forced, formidable,
   gargantuan, giant, giantlike, gigantic, grueling, hairy, hard,
   hard-earned, hard-fought, heavy, hefty, heroic, huge, immense,
   infinite, intricate, jawbreaking, jumbo, killing, knotted, knotty,
   labored, laborious, mammoth, mean, mighty, monster, monstrous,
   monumental, mountainous, no picnic, not easy, onerous, operose,
   oppressive, painful, prodigious, profound, punishing, rigorous,
   rough, rugged, set with thorns, severe, spiny, steep, strained,
   strenuous, stupendous, thorny, ticklish, titanic, toilsome, tough,
   towering, tremendous, tricky, troublesome, uphill, vast, wearisome,
   wicked