The Collaborative International Dictionary of English v.0.48:

Further \Fur"ther\, adv. [A comparative of forth; OE. further,
   forther, AS. fur?or, far?ur; akin to G. f["u]rder. See
   Forth, adv.]
   To a greater distance; in addition; moreover. See Farther.
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         Carries us, I know not how much further, into familiar
         company.                                 --M. Arnold.
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         They sdvanced us far as Eleusis and Thria; but no
         further.                                 --Jowett
                                                  (Thucyd. ).
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   Further off, not so near; apart by a greater distance.
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The Collaborative International Dictionary of English v.0.48:

Further \Fur"ther\, a. compar. [Positive wanting; superl.
   Furthest.]
   1. More remote; at a greater distance; more in advance;
      farther; as, the further end of the field. See Farther.
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   2. Beyond; additional; as, a further reason for this opinion;
      nothing further to suggest.
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   Note: The forms further and farther are in general not
         differentiated by writers, but further is preferred by
         many when application to quantity or degree is implied.
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The Collaborative International Dictionary of English v.0.48:

Further \Fur"ther"\, v. t. [imp. & p. p. Furthered; p. pr. &
   vb. n. Furthering.] [OE. furthren, forthren, AS.
   fyr[eth]ran, fyr[eth]rian. See Further, adv.]
   To help forward; to promote; to advance; to forward; to help
   or assist.
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         This binds thee, then, to further my design. --Dryden.
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         I should nothing further the weal public. --Robynsom
                                                  (More's
                                                  Utopia).
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WordNet (r) 3.0 (2006):

further
    adv 1: to or at a greater extent or degree or a more advanced
           stage (`further' is used more often than `farther' in
           this abstract sense); "further complicated by uncertainty
           about the future"; "let's not discuss it further";
           "nothing could be further from the truth"; "they are
           further along in their research than we expected"; "the
           application of the law was extended farther"; "he is
           going no farther in his studies" [syn: further,
           farther]
    2: in addition or furthermore; "if we further suppose"; "stated
       further that he would not cooperate with them"; "they are
       definitely coming; further, they should be here already"
    3: to or at a greater distance in time or space (`farther' is
       used more frequently than `further' in this physical sense);
       "farther north"; "moved farther away"; "farther down the
       corridor"; "the practice may go back still farther to the
       Druids"; "went only three miles further"; "further in the
       future" [syn: farther, further]
    adj 1: more distant in especially degree; "nothing could be
           further from the truth"; "further from our expectations";
           "farther from the truth"; "farther from our expectations"
           [syn: further, farther]
    v 1: promote the growth of; "Foster our children's well-being
         and education" [syn: foster, further]
    2: contribute to the progress or growth of; "I am promoting the
       use of computers in the classroom" [syn: promote,
       advance, boost, further, encourage]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "further":
   above, accessory, added, additional, additionally, advance,
   advantage, again, aid, all included, also, altogether,
   among other things, ancillary, and all, and also, and so, another,
   as well, assist, au reste, auxiliary, back, beside, besides,
   beyond, boost, collateral, conduce to, contribute to, contributory,
   else, en plus, encourage, engender, expedite, extra, facilitate,
   farther, favor, for lagniappe, forward, foster, fresh, furthermore,
   generate, hasten, help, in addition, inter alia, into the bargain,
   item, lend wings to, likewise, make for, more, more distant,
   moreover, new, on the side, on top of, other, over, over and above,
   patronize, plus, promote, propagate, push forward, put forward,
   quicken, remoter, renewed, serve, set forward, similarly, spare,
   speed, supernumerary, supplemental, supplementary, support,
   surplus, then, therewith, thither, to boot, too, ulterior, yet,
   yon, yonder