The Collaborative International Dictionary of English v.0.48:

Apropos \Ap"ro*pos`\ ([a^]p"r[-o]*p[=o]`), a. & adv. [F. [`a]
   propos; [`a] (L. ad) + propos purpose, L. proposium plan,
   purpose, fr. proponere to propose. See Propound.]
   1. Opportunely or opportune; seasonably or seasonable.
      [1913 Webster]

            A tale extremely apropos.             --Pope.
      [1913 Webster]

   2. By the way; to the purpose; suitably to the place or
      subject; -- a word used to introduce an incidental
      observation, suited to the occasion, though not strictly
      belonging to the narration.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apropos
    adv 1: at an opportune time; "your letter arrived apropos" [syn:
           seasonably, timely, well-timed, apropos]
    2: 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]
    adj 1: of an appropriate or pertinent nature [ant: malapropos]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "apropos":
   a propos, about, ad rem, adapted, admissible, against, anent,
   appertaining, applicable, applying, apposite, appropriate,
   apropos of, apt, as for, as regards, as respects, as to, becoming,
   befitting, belonging, by the by, by the way, concerning,
   dovetailing, en passant, felicitous, fit, fitted, fitting,
   for example, geared, germane, happy, in passing, in point, in re,
   in reference to, in respect to, incidentally, involving,
   just right, likely, material, meet, meshing, on the button,
   opportune, par exemple, parenthetically, pat, pertaining,
   pertinent, proper, qualified, re, regarding, relevant, respecting,
   right, seasonable, sortable, speaking of, suitable, suited,
   suiting, tailored, to the point, to the purpose, touching, toward,
   with respect to