The Collaborative International Dictionary of English v.0.48:

Occasionally \Oc*ca"sion*al*ly\, adv.
   In an occasional manner; on occasion; at times, as
   convenience requires or opportunity offers; not regularly.
   --Stewart.
   [1913 Webster]

         The one, Wolsey, directly his subject by birth; the
         other, his subject occasionally by his preferment.
                                                  --Fuller.
   [1913 Webster]

WordNet (r) 3.0 (2006):

occasionally
    adv 1: now and then or here and there; "he was arrogant and
           occasionally callous"; "open areas are only occasionally
           interrupted by clumps of trees"; "they visit New York on
           occasion"; "now and again she would take her favorite
           book from the shelf and read to us"; "as we drove along,
           the beautiful scenery now and then attracted his
           attention" [syn: occasionally, on occasion, once in
           a while, now and then, now and again, at times,
           from time to time]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "occasionally":
   at infrequent intervals, at intervals, at odd times, at times,
   at various times, betweentimes, betweenwhiles, brokenly,
   by catches, by fits, by jerks, by skips, by snatches,
   disconnectedly, discontinuously, every so often, fitfully,
   haphazardly, here and there, in spots, infrequently,
   intermittently, irregularly, now and then, off and on,
   on divers occasions, on occasion, once and again,
   only occasionally, only when necessary, per saltum, periodically,
   randomly, skippingly, sometimes, sporadically, uncommonly,
   willy-nilly