The Collaborative International Dictionary of English v.0.48:

Far \Far\, a. [Fartherand Farthestare used as the compar.
   and superl. of far, although they are corruptions arising
   from confusion with further and furthest. See Further.]
   [OE. fer, feor, AS. feor; akin to OS. fer, D. ver, OHG.
   ferro, adv., G. fern, a., Icel. fjarri, Dan. fjirn, Sw.
   fjerran, adv., Goth. fa[imac]rra, adv., Gr. ????? beyond,
   Skr. paras, adv., far, and prob. to L. per through, and E.
   prefix for-, as in forgive, and also to fare. Cf. Farther,
   Farthest.]
   1. Distant in any direction; not near; remote; mutually
      separated by a wide space or extent.
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            They said, . . . We be come from a far country.
                                                  --Josh. ix. 6.
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            The nations far and near contend in choice.
                                                  --Dryden.
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   2. Remote from purpose; contrary to design or wishes; as, far
      be it from me to justify cruelty.
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   3. Remote in affection or obedience; at a distance, morally
      or spiritually; t enmity with; alienated.
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            They that are far from thee ahsll perish. --Ps.
                                                  lxxiii. 27.
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   4. Widely different in nature or quality; opposite in
      character.
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            He was far from ill looking, though he thought
            himself still farther.                --F. Anstey.
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   5. The more distant of two; as, the far side (called also off
      side) of a horse, that is, the right side, or the one
      opposite to the rider when he mounts.
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   Note: The distinction between the adjectival and adverbial
         use of far is sometimes not easily discriminated.
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   By far, by much; by a great difference.

   Far between, with a long distance (of space or time)
      between; at long intervals. "The examinations are few and
      far between." --Farrar.
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The Collaborative International Dictionary of English v.0.48:

Farther \Far"ther\ (f[aum]r"[th][~e]r), a., compar. of Far.
   [superl. Farthest (-[th][e^]st). See Further.] [For
   farrer, OE. ferrer, compar. of far; confused with further.
   Cf. Farthest.]
   1. More remote; more distant than something else.
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   2. Tending to a greater distance; beyond a certain point;
      additional; further.
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            Before our farther way the fates allow. --Dryden.
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            Let me add a farther Truth.           --Dryden.
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            Some farther change awaits us.        --MIlton.
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The Collaborative International Dictionary of English v.0.48:

Farthest \Far"thest\ adv.
   At or to the greatest distance. See Furthest.
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The Collaborative International Dictionary of English v.0.48:

Farthest \Far"thest\ (f[aum]r"[th][e^]st), a. Superl. of far.
   [See Farther and cf. Furthest]
   Most distant or remote; as, the farthest degree. See
   Furthest.
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WordNet (r) 3.0 (2006):

farthest
    adv 1: to the greatest distance in space or time (`farthest' is
           used more often than `furthest' in this physical sense);
           "see who could jump the farthest"; "chose the farthest
           seat from the door"; "he swam the furthest" [syn:
           farthest, furthest]
    2: to the greatest degree or extent or most advanced stage
       (`furthest' is used more often than `farthest' in this
       abstract sense); "went the furthest of all the children in
       her education"; "furthest removed from reality"; "she goes
       farthest in helping us" [syn: furthest, farthest]
    adj 1: (comparatives of `far') most remote in space or time or
           order; "had traveled to the farthest frontier"; "don't go
           beyond the farthermost (or furthermost) tree"; "explored
           the furthest reaches of space"; "the utmost tip of the
           peninsula" [syn: farthermost, farthest,
           furthermost, furthest, utmost, uttermost]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "farthest":
   boundary, caudal, conclusive, definitive, determinative, endmost,
   eventual, extreme, farthermost, farthest off, final, furthermost,
   furthest, last, limiting, most distant, outermost, outmost, polar,
   remotest, tail, terminal, terminating, terminative, ultimate,
   utmost, uttermost