The Collaborative International Dictionary of English v.0.48:

Melting pot \Melt"ing pot\ n.
   1. A vessel in which anything is melted; a crucible.
      [1913 Webster]

   2. (Sociology) (fig.) A place where people of different
      backgrounds become similar in culture. The United States
      has often been referred to as a melting pot, though the
      differences in cultures of recently arrived immigrants
      persists beyond the generation of immigrants.
      [PJC]

WordNet (r) 3.0 (2006):

melting pot
    n 1: an environment in which many ideas and races are socially
         assimilated
    2: a vessel made of material that does not melt easily; used for
       high temperature chemical reactions [syn: crucible,
       melting pot]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "melting pot":
   America, Columbia, Land of Liberty, US, USA, Uncle Sugar,
   United States, Yankeeland, agitator, alembic, anvil, beater,
   blender, caldron, cement mixer, churn, crucible, eggbeater,
   emulsifier, engine, lathe, machine, mixer, mortar, motor, retort,
   stateside, test tube, the States, the melting pot, transducer,
   transformer