The Collaborative International Dictionary of English v.0.48:

Mixer \Mix"er\, n.
   1. One who, or that which, mixes.
      [1913 Webster]

   2. A person who has social intercourse with others of many
      sorts; a person viewed as to his casual sociability; --
      commonly used with some characterizing adjective; as, a
      good mixer; a bad mixer. [Colloq. or Slang, U. S.]
      [Webster 1913 Suppl.]

   3. a social gathering, game, or dance organized to provide an
      opportunity for people to meet each other; as, on the
      first night of the conference they had a wine-and-cheese
      mixer.
      [PJC]

   4. a nonalcoholic beverage (such as fruit juice, club soda or
      ginger ale) added to an alcoholic beverage to produce a
      mixed drink.
      [PJC]

   5. any device used for mixing.
      [PJC]

   6. an electronic device for blending or manipulating sounds
      from different sources to produce a composite soundtrack,
      for an audio recording, video recording, or a movie.
      [PJC]

   7. The technician who operates a mixer[6].
      [PJC]

WordNet (r) 3.0 (2006):

mixer
    n 1: a party of people assembled to promote sociability and
         communal activity [syn: sociable, social, mixer]
    2: club soda or fruit juice used to mix with alcohol
    3: electronic equipment that mixes two or more input signals to
       give a single output signal
    4: a kitchen utensil that is used for mixing foods

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "mixer":
   ARRL, agitator, amateur radio operator, bal, bal costume,
   bal masque, ball, barn dance, beater, blender, bon vivant,
   cement mixer, churn, clubman, clubwoman, console, control desk,
   control engineer, control panel, country dance, crucible, dance,
   eggbeater, emulsifier, excellent companion, fancy-dress ball,
   good company, good mixer, ham, hop, jack field, joiner,
   man-about-town, mask, masked ball, masque, masquerade,
   masquerade ball, master control desk, melting pot, monitor,
   nightclub habitue, playboy, pleasant company, prom, promenade,
   radio electrician, radio engineer, radio operator,
   radio technician, radioman, radiotelegrapher, radiotrician,
   record hop, shindig, shindy, social lion, square dance, stag dance,
   tea dance, the dansant