The Collaborative International Dictionary of English v.0.48:

Molehill \Mole"hill`\, n.
   A little hillock of earth thrown up by moles working under
   ground; hence, a very small hill, or an insignificant
   obstacle or difficulty; as, to make a mountain out of a
   molehill.
   [1913 Webster]

         Having leapt over such mountains, lie down before a
         molehill.                                --South.
   [1913 Webster] mMolendinaceous

WordNet (r) 3.0 (2006):

molehill
    n 1: a mound of earth made by moles while burrowing

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "molehill":
   a continental, a curse, a damn, a darn, a hoot, anthill, bagatelle,
   bank, barrow, bauble, bean, bibelot, bit, brae, brass farthing,
   butte, button, cent, cock, curio, down, drift, drumlin, dune,
   embankment, farce, farthing, feather, fell, fig, fleabite,
   folderol, foothills, fribble, frippery, gaud, gewgaw, gimcrack,
   hair, halfpenny, haycock, haymow, hayrick, haystack, heap, hill,
   hill of beans, hillock, hummock, jest, joke, kickshaw, knickknack,
   knickknackery, knob, knoll, minikin, mockery, monticle, monticule,
   moor, mound, mow, peppercorn, picayune, pile, pin, pinch of snuff,
   pinprick, pyramid, rap, red cent, rick, row of pins, rush,
   sand dune, shit, snap, sneeshing, snowdrift, sou, stack, straw,
   swell, toy, trifle, trinket, triviality, tuppence, two cents,
   twopence, whim-wham