The Collaborative International Dictionary of English v.0.48:

Bellwether \Bell"weth`er\, n.
   1. A wether, or sheep, which leads the flock, with a bell on
      his neck.
      [1913 Webster]

   2. Hence: A leader. [Contemptuous] --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bellwether
    n 1: someone who assumes leadership of a movement or activity
    2: sheep that leads the herd often wearing a bell

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "bellwether":
   Mahdi, ancestor, announcer, antecedent, avant-garde, bell cow,
   bell mare, born leader, buccinator, bushwhacker,
   charismatic leader, choirmaster, choragus, conductor, coryphaeus,
   doyen, duce, ewe, ewe lamb, explorer, file leader, forebear,
   foregoer, forerunner, front runner, frontiersman, fugleman,
   groundbreaker, guide, harbinger, herald, innovator,
   inspired leader, jumbuck, lamb, lambkin, lead, lead runner, leader,
   leader of men, messenger, messiah, mutton, pacemaker, pacesetter,
   pathfinder, pilot, pioneer, point, precedent, precentor, precursor,
   predecessor, ram, ringleader, scout, sheep, standard-bearer,
   stormy petrel, symphonic conductor, teg, torchbearer, trailblazer,
   trailbreaker, tup, vanguard, vaunt-courier, voortrekker, wether,
   yeanling