WordNet (r) 3.0 (2006):

superpower
    n 1: a state powerful enough to influence events throughout the
         world [syn: world power, major power, great power,
         power, superpower]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "superpower":
   ally, amperage, archduchy, archdukedom, armipotence, authority,
   beef, black power, body politic, brute force, buffer state,
   captive nation, charge, charisma, chieftaincy, chieftainry,
   city-state, clout, cogence, cogency, colony, commonweal,
   commonwealth, compulsion, country, county, dint, domain, dominion,
   drive, duchy, dukedom, duress, earldom, effect, effectiveness,
   effectuality, empery, empire, energy, flower power, force,
   force majeure, forcefulness, free city, full blast, full force,
   grand duchy, influence, kingdom, land, main force, main strength,
   mana, mandant, mandate, mandated territory, mandatee, mandatory,
   might, might and main, mightiness, moxie, muscle power, nation,
   nationality, pizzazz, polis, polity, poop, possession, potence,
   potency, potentiality, power, power pack, power structure,
   power struggle, powerfulness, prepotency, principality, principate,
   productiveness, productivity, protectorate, province, puissance,
   pull, punch, puppet government, puppet regime, push, realm,
   republic, satellite, seneschalty, settlement, sinew,
   sovereign nation, state, steam, strength, strong arm, sultanate,
   superiority, territory, toparchia, toparchy, validity, vehemence,
   vigor, vim, virility, virtue, virulence, vitality, wattage,
   weight