The Collaborative International Dictionary of English v.0.48:

Either \Ei"ther\ ([=e]"[th][~e]r or [imac]"[th][~e]r; 277), a. &
   pron. [OE. either, aither, AS. [=ae]g[eth]er,
   [=ae]ghw[ae][eth]er (akin to OHG. [=e]ogiwedar, MHG.
   iegeweder); [=a] + ge + hw[ae][eth]er whether. See Each,
   and Whether, and cf. Or, conj.]
   1. One of two; the one or the other; -- properly used of two
      things, but sometimes of a larger number, for any one.
      [1913 Webster]

            Lepidus flatters both,
            Of both is flattered; but he neither loves,
            Nor either cares for him.             --Shak.
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            Scarce a palm of ground could be gotten by either of
            the three.                            --Bacon.
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            There have been three talkers in Great British,
            either of whom would illustrate what I say about
            dogmatists.                           --Holmes.
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   2. Each of two; the one and the other; both; -- formerly,
      also, each of any number.
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            His flowing hair
            In curls on either cheek played.      --Milton.
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            On either side . . . was there the tree of life.
                                                  --Rev. xxii.
                                                  2.
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            The extreme right and left of either army never
            engaged.                              --Jowett
                                                  (Thucyd).
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The Collaborative International Dictionary of English v.0.48:

Either \Ei"ther\, conj.
   Either precedes two, or more, co["o]rdinate words or phrases,
   and is introductory to an alternative. It is correlative to
   or.
   [1913 Webster]

         Either he is talking, or he is pursuing, or he is in a
         journey, or peradventure he sleepeth.    --1 Kings
                                                  xviii. 27.
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         Few writers hesitate to use either in what is called a
         triple alternative; such as, We must either stay where
         we are, proceed, or recede.              --Latham.
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   Note: Either was formerly sometimes used without any
         correlation, and where we should now use or.
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               Can the fig tree, my brethren, bear olive
               berries? either a vine, figs?      --James iii.
                                                  12.
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WordNet (r) 3.0 (2006):

either
    adv 1: after a negative statement used as an intensive meaning
           something like `likewise' or `also'; "he isn't stupid,
           but he isn't exactly a genius either"; "I don't know
           either"; "if you don't order dessert I won't either"

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "either":
   a certain, a deux, an, any, any one, anybody, anyone, anything,
   atomic, aught, both, correspondingly, exclusive, for two,
   identically, in kind, in like manner, in that way, individual,
   indivisible, integral, irreducible, like, like that, like this,
   likewise, lone, monadic, monistic, one, similarly, simple, single,
   singular, so, sole, solid, solitary, tete-a-tete, the two, thus,
   unanalyzable, undivided, uniform, unique, unitary, whole