The Collaborative International Dictionary of English v.0.48:

Incidental \In`ci*den"tal\, a.
   Happening, as an occasional event, without regularity; coming
   without design; casual; accidental; hence, not of prime
   concern; subordinate; collateral; as, an incidental
   conversation; an incidental occurrence; incidental expenses.
   [1913 Webster]

         By some, religious duties . . . appear to be regarded .
         . . as an incidental business.           --Rogers.

   Syn: Accidental; casual; fortuitous; contingent; chance;
        collateral. See Accidental. -- In`ci*den"tal*ly,
        adv. -- In`ci*den"tal*ness, n.
        [1913 Webster]

              I treat either or incidentally of colors. --Boyle.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incidentally
    adv 1: introducing a different topic; in point of fact;
           "incidentally, I won't go to the party" [syn: by the
           way, by the bye, incidentally, apropos]
    2: of a minor or subordinate nature; "these magnificent
       achievements were only incidentally influenced by Oriental
       models" [syn: incidentally, accidentally]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "incidentally":
   a propos, accidentally, apropos, apropos of, as it chanced,
   by a fluke, by accident, by chance, by good fortune, by hazard,
   by the by, by the way, casually, en passant, for example,
   for some reason, fortuitously, in passing, in some way,
   par exemple, parenthetically, perchance, somehow, somehow or other,
   speaking of, unpredictably