The Collaborative International Dictionary of English v.0.48:

Now \Now\ (nou), adv. [OE. nou, nu, AS. n[=u], nu; akin to D.,
   OS., & OHG. nu, G. nu, nun, Icel., n[=u], Dan., Sw., & Goth.
   nu, L. nunc, Gr. ny`, ny^n, Skr. nu, n[=u]. [root]193. Cf.
   New.]
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   1. At the present time; at this moment; at the time of
      speaking; instantly; as, I will write now.
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            I have a patient now living, at an advanced age, who
            discharged blood from his lungs thirty years ago.
                                                  --Arbuthnot.
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   2. Very lately; not long ago.
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            They that but now, for honor and for plate,
            Made the sea blush with blood, resign their hate.
                                                  --Waller.
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   3. At a time contemporaneous with something spoken of or
      contemplated; at a particular time referred to.
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            The ship was now in the midst of the sea. --Matt.
                                                  xiv. 24.
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   4. In present circumstances; things being as they are; --
      hence, used as a connective particle, to introduce an
      inference or an explanation.
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            How shall any man distinguish now betwixt a parasite
            and a man of honor?                   --L'Estrange.
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            Why should he live, now nature bankrupt is? --Shak.
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            Then cried they all again, saying, Not this man, but
            Barabbas. Now, Barabbas was a robber. --John xviii.
                                                  40.
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            The other great and undoing mischief which befalls
            men is, by their being misrepresented. Now, by
            calling evil good, a man is misrepresented to others
            in the way of slander.                --South.
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   Now and again, now and then; occasionally.

   Now and now, again and again; repeatedly. [Obs.] --Chaucer.

   Now and then, at one time and another; indefinitely;
      occasionally; not often; at intervals. "A mead here, there
      a heath, and now and then a wood." --Drayton.

   Now now, at this very instant; precisely now. [Obs.] "Why,
      even now now, at holding up of this finger, and before the
      turning down of this." --J. Webster (1607).

   Now . . . now, alternately; at one time . . . at another
      time. "Now high, now low, now master up, now miss."
      --Pope.
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The Collaborative International Dictionary of English v.0.48:

Then \Then\ ([th][e^]n), adv. [Originally the same word as than.
   See Than.]
   1. At that time (referring to a time specified, either past
      or future).
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            And the Canaanite was then in the land. --Gen. xii.
                                                  6.
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            Now I know in part; but then shall I know even as
            also I am known.                      --1 Cor. xiii.
                                                  12.
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   2. Soon afterward, or immediately; next; afterward.
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            First be reconciled to thy brother, and then come
            and offer thy gift.                   --Matt. v. 24.
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   3. At another time; later; again.
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            One while the master is not aware of what is done,
            and then in other cases it may fall out to be own
            act.                                  --L'Estrange.
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   By then.
      (a) By that time.
      (b) By the time that. [Obs.]
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                But that opinion, I trust, by then this
                following argument hath been well read, will be
                left for one of the mysteries of an indulgent
                Antichrist.                       --Milton.
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   Now and then. See under Now, adv.

   Till then, until that time; until the time mentioned.
      --Milton.
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   Note: Then is often used elliptically, like an adjective, for
         then existing; as, the then administration.
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WordNet (r) 3.0 (2006):

now and then
    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:

20 Moby Thesaurus words for "now and then":
   at infrequent intervals, at intervals, at odd times, at times,
   at various times, betimes, betweentimes, betweenwhiles,
   ever and again, ever and anon, every so often, here and there, now,
   occasionally, on divers occasions, on occasion, once and again,
   only occasionally, only when necessary, sometimes