The Collaborative International Dictionary of English v.0.48:

Organizer \Or"gan*i`zer\, n.
   One who organizes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

organizer
    n 1: a person who brings order and organization to an
         enterprise; "she was the organizer of the meeting" [syn:
         organizer, organiser, arranger]
    2: someone who enlists workers to join a union [syn:
       organizer, organiser, labor organizer]
    3: a lightweight consumer electronic device that looks like a
       hand-held computer but instead performs specific tasks; can
       serve as a diary or a personal database or a telephone or an
       alarm clock etc. [syn: personal digital assistant, PDA,
       personal organizer, personal organiser, organizer,
       organiser]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "organizer":
   a, ancestors, apprentice, architect, artificer, artist, author,
   bargainer, begetter, beginner, builder, business agent, cardholder,
   conceiver, constructor, contriver, craftsman, creator, designer,
   developer, deviser, discoverer, effector, engenderer, engineer,
   enterpriser, entrepreneur, executor, executrix, father, founder,
   framer, generator, grower, inaugurator, industrialist, initiator,
   instigator, institutor, introducer, inventor, journeyman,
   labor organizer, maker, manufacturer, master, master craftsman,
   mother, negotiator, originator, past master, planner, precursor,
   prime mover, producer, projector, promoter, raiser, realizer,
   shaper, shop steward, sire, smith, strategian, strategist,
   tactician, trade unionist, union member, union officer, unionist,
   wright