The Collaborative International Dictionary of English v.0.48:

Beside \Be*side"\, prep. [OE. biside, bisiden, bisides, prep.
   and adv., beside, besides; pref. be- by + side. Cf. Besides,
   and see Side, n.]
   1. At the side of; on one side of. "Beside him hung his bow."
      --Milton.
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   2. Aside from; out of the regular course or order of; in a
      state of deviation from; out of.
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            [You] have done enough
            To put him quite beside his patience. --Shak.
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   3. Over and above; distinct from; in addition to.

   Note: [In this use besides is now commoner.]
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               Wise and learned men beside those whose names are
               in the Christian records.          --Addison.
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   To be beside one's self, to be out of one's wits or senses.
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            Paul, thou art beside thyself.        --Acts xxvi.
                                                  24.
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   Syn: Beside, Besides.

   Usage: These words, whether used as prepositions or adverbs,
          have been considered strictly synonymous, from an
          early period of our literature, and have been freely
          interchanged by our best writers. There is, however, a
          tendency, in present usage, to make the following
          distinction between them: 1. That beside be used only
          and always as a preposition, with the original meaning
          "by the side of; " as, to sit beside a fountain; or
          with the closely allied meaning "aside from", "apart
          from", or "out of"; as, this is beside our present
          purpose; to be beside one's self with joy. The
          adverbial sense to be wholly transferred to the
          cognate word. 2. That besides, as a preposition, take
          the remaining sense "in addition to", as, besides all
          this; besides the considerations here offered. "There
          was a famine in the land besides the first famine."
          --Gen. xxvi. 1. And that it also take the adverbial
          sense of "moreover", "beyond", etc., which had been
          divided between the words; as, besides, there are
          other considerations which belong to this case. The
          following passages may serve to illustrate this use of
          the words:

                Lovely Thais sits beside thee.    --Dryden.
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                Only be patient till we have appeased
                The multitude, beside themselves with fear.
                                                  --Shak.
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                It is beside my present business to enlarge on
                this speculation.                 --Locke.
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                Besides this, there are persons in certain
                situations who are expected to be charitable.
                                                  --Bp. Porteus.
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                And, besides, the Moor
                May unfold me to him; there stand I in much
                peril.                            --Shak.
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                That man that does not know those things which
                are of necessity for him to know is but an
                ignorant man, whatever he may know besides.
                                                  --Tillotson.
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   Note: See Moreover.
         [1913 Webster] Besides

The Collaborative International Dictionary of English v.0.48:

Besides \Be*sides"\, Beside \Be*side"\, adv. [OE. Same as
   beside, prep.; the ending -s is an adverbial one, prop. a
   genitive sign.]
   1. On one side. [Obs.] --Chaucer. Shak.
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   2. More than that; over and above; not included in the
      number, or in what has been mentioned; moreover; in
      addition.
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            The men said unto Lot, Hast thou here any besides ?
                                                  --Gen. xix.
                                                  12.
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            To all beside, as much an empty shade,
            An Eugene living, as a C[ae]sar dead. --Pope.
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   Note: These sentences may be considered as elliptical.
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Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "beside":
   abeam, above, abreast, additionally, again, agitated, all included,
   along by, alongside, also, altogether, among other things, and all,
   and also, and so, apart from, as compared with, as well, aside,
   aside from, au reste, away from, bar, barring, besides, beyond,
   but, by, by comparison with, close to, compared with, crazy, else,
   en plus, ex, except, except for, excepting, excluding,
   exclusive of, extra, farther, for lagniappe, further, furthermore,
   hard by, in addition, in apposition, in comparison with,
   in conjunction, in juxtaposition, inter alia, into the bargain,
   item, leaving out, let alone, likewise, mad, more, moreover, near,
   nearby, next to, nigh, off, omitting, on the side, on top of,
   opposite, outside of, over, over against, over and above,
   overwrought, plus, precluding, round, save, save and except,
   saving, similarly, taken with, than, then, therewith, to boot, too,
   unless, upset, with, without, yet