The Collaborative International Dictionary of English v.0.48:

Nemesis \Nem"e*sis\ (n[e^]m"[-e]*s[i^]s), prop. n. [L., fr. Gr.
   Ne`mesis, orig., distribution, fr. ne`mein to distribute. See
   Nomad.] (Class. Myth.)
   The goddess of retribution or vengeance; hence, retributive
   justice personified; divine vengeance.
   [1913 Webster]

         This is that ancient doctrine of nemesis who keeps
         watch in the universe, and lets no offense go
         unchastised.                             --Emerson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Nemesis
    n 1: (Greek mythology) the goddess of divine retribution and
         vengeance
    2: something causing misery or death; "the bane of my life"
       [syn: bane, curse, scourge, nemesis]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "nemesis":
   affliction, bane, bugbear, burden, calamity, castigation,
   chastening, chastisement, condign punishment, correction,
   crushing burden, curse, death, defensibility, deserts, destruction,
   disciplinary measures, discipline, disease, drumhead justice,
   dueness, equitableness, equity, evenhandedness, evil, ferule,
   give-and-take, grievance, harm, infliction, judgment,
   judicial punishment, justice, justifiability, justifiableness,
   justification, justness, lawfulness, legality, measure for measure,
   meetness, open wound, pains, pains and punishments, pay, payment,
   penal retribution, penalty, penology, pest, pestilence, plague,
   poetic justice, properness, propriety, punishment, punition,
   retribution, retributive justice, right, rightfulness, rightness,
   rude justice, running sore, scales of justice, scourge,
   summary justice, thorn, torment, vexation, visitation,
   warrantability, warrantedness, well-deserved punishment,
   what is right, what-for, woe



Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "Nemesis":
   Alecto, Astraea, Dike, Jupiter Fidius, Justice, Justitia, Loki,
   Megaera, Minos, Rhadamanthus, Set, Themis, Tisiphone, Typhon,
   avenger, blindfolded Justice, burning rage, furious rage, furor,
   fury, passion, rage, revanchist, tearing passion, the Erinyes,
   the Eumenides, the Furies, towering rage, vehemence, vindicator,
   violence