WordNet (r) 3.0 (2006):

rube
    n 1: a person who is not very intelligent or interested in
         culture [syn: yokel, rube, hick, yahoo, hayseed,
         bumpkin, chawbacon]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "rube":
   babe, backwoodsman, boor, boorish, bucolic, bumpkin, butterfingers,
   child, child of nature, churlish, clod, cloddish, clodhopper,
   clodhopping, clown, clownish, clumsy, countrified, country bumpkin,
   country-born, country-bred, dove, dub, duffer, dupe, farmer,
   farmerish, foozler, foul-up, from the sticks, fumble-fist, gawk,
   gawky, goof, goofball, goofer, hayseed, hick, hicky, hillbilly,
   hobnailed, infant, ingenue, innocent, inurbane, lamb, looby, lout,
   loutish, lubber, lummox, lump, lumpen, lumpish, mere child, muff,
   muffer, noble savage, oaf, provincial, redneck, rustic,
   simple soul, slob, stumblebum, stumblebunny, uncouth, uncultivated,
   uncultured, unpolished, unrefined, unsophisticate, up-country,
   yokel, yokelish