The Collaborative International Dictionary of English v.0.48:

Fortuitous \For*tu"i*tous\, a. [L. fortuitus; akin to forte,
   adv., by chance, prop. abl. of fors, fortis, chance. See
   Fortune.]
   1. Happening by chance; coming or occuring unexpectedly, or
      without any known cause; chance; as, the fortuitous
      concourse of atoms.
      [1913 Webster]

            It was from causes seemingly fortuitous . . . that
            all the mighty effects of the Reformation flowed.
                                                  --Robertson.
      [1913 Webster]

            So as to throw a glancing and fortuitous light upon
            the whole.                            --Hazlitt.
      [1913 Webster]

   2. (LAw) Happening independently of human will or means of
      foresight; resulting from unavoidable physical causes.
      --Abbott.

   Syn: Accidental; casual; contingent; incidental. See
        Accidental. -- For*tu"i*tous*ly, adv. --
        For*tu"i*tous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fortuitous
    adj 1: having no cause or apparent cause; "a causeless miracle";
           "fortuitous encounters--strange accidents of fortune";
           "we cannot regard artistic invention as...uncaused and
           unrelated to the times" [syn: causeless, fortuitous,
           uncaused]
    2: occurring by happy chance; "profits were enhanced by a
       fortuitous drop in the cost of raw materials"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "fortuitous":
   accessory, accidental, additional, adscititious, adventitious,
   aleatory, appurtenant, ascititious, auxiliary, casual, causeless,
   chance, chancy, circumstantial, collateral, contingent, destinal,
   dicey, extra, fatal, fatidic, fluky, iffy, incidental,
   indeterminate, inessential, nonessential, odd, risky, secondary,
   subsidiary, superadded, superfluous, supervenient, supplemental,
   supplementary, uncaused, undetermined, unessential, unexpected,
   unforeseeable, unforeseen, unlooked-for, unpredictable