WordNet (r) 3.0 (2006):

stepping stone
    n 1: a stone in a marsh or shallow water that can be stepped on
         in crossing
    2: any means of advancement; "the job was just a stepping stone
       on his way to fame and riches"

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "stepping stone":
   Bifrost, a leg up, back stairs, bascule bridge, bateau bridge,
   bridge, cantilever bridge, catwalk, chance, clear stage, companion,
   companionway, doorstep, drawbridge, escalier, fair field,
   fair game, fire escape, flight of steps, floating bridge, flyover,
   footbridge, footrest, footstep, gangboard, gangplank, gangway,
   incline, landing, landing stage, liberty, lift bridge, occasion,
   opening, opportunism, opportunity, overbridge, overcrossing,
   overpass, perron, place, pontoon bridge, ramp, rest, riser, room,
   rope bridge, round, rundle, rung, scale, scope, span,
   spiral staircase, spoke, stair, staircase, stairs, stairway, stave,
   step, step stool, stepping-stones, steps, stepstone, stile, string,
   suspension bridge, swing bridge, time, toll bridge, tread,
   treads and risers, viaduct