WordNet (r) 3.0 (2006):

groundbreaker
    n 1: someone who helps to open up a new line of research or
         technology or art [syn: pioneer, innovator,
         trailblazer, groundbreaker]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "groundbreaker":
   ancestor, announcer, antecedent, avant-garde, bellwether,
   buccinator, bushwhacker, explorer, forebear, foregoer, forerunner,
   front runner, frontiersman, fugleman, guide, harbinger, herald,
   innovator, lead runner, leader, messenger, pathfinder, pioneer,
   point, precedent, precursor, predecessor, scout, stormy petrel,
   trailblazer, trailbreaker, vanguard, vaunt-courier, voortrekker