The Collaborative International Dictionary of English v.0.48:

Explorer \Ex*plor"er\, n.
   One who explores; also, an apparatus with which one explores,
   as a diving bell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

explorer
    n 1: someone who travels into little known regions (especially
         for some scientific purpose) [syn: explorer,
         adventurer]
    2: a commercial browser [syn: Internet Explorer, Explorer,
       IE]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "explorer":
   adventurer, alpinist, ancestor, announcer, antecedent, astronaut,
   avant-garde, bellwether, buccinator, bushwhacker, camper, climber,
   comers and goers, commuter, cosmopolite, cruiser, excursionist,
   fare, forebear, foregoer, forerunner, front runner, frontiersman,
   fugleman, globe-girdler, globe-trotter, goer, groundbreaker, guide,
   hajji, harbinger, herald, innovator, jet set, jet-setter,
   journeyer, lead runner, leader, mariner, messenger, mountaineer,
   palmer, passenger, passerby, pathfinder, pilgrim, pioneer, point,
   precedent, precursor, predecessor, rubberneck, rubbernecker,
   sailor, scout, sightseer, stormy petrel, straphanger, tourer,
   tourist, trailblazer, trailbreaker, transient, traveler, trekker,
   tripper, vanguard, vaunt-courier, viator, visiting fireman,
   voortrekker, voyager, voyageur, wayfarer, world-traveler