The Collaborative International Dictionary of English v.0.48:

Truly \Tru"ly\, adv. [From True.]
   1. In a true manner; according to truth; in agreement with
      fact; as, to state things truly; the facts are truly
      represented.
      [1913 Webster]

            I can not truly say how I came here.  --Shak.
      [1913 Webster]

   2. Exactly; justly; precisely; accurately; as, to estimate
      truly the weight of evidence.
      [1913 Webster]

   3. Sincerely; honestly; really; faithfully; as, to be truly
      attached to a lover; the citizens are truly loyal to their
      prince or their country. --Burke.
      [1913 Webster]

   4. Conformably to law; legally; legitimately.
      [1913 Webster]

            His innocent babe [is] truly begotten. --Shak.
      [1913 Webster]

   5. In fact; in deed; in reality; in truth.
      [1913 Webster]

            Beauty is excelled by manly grace
            And wisdom, which alone is truly fair. --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

truly
    adv 1: in accordance with truth or fact or reality; "she was now
           truly American"; "a genuinely open society"; "they don't
           really listen to us" [syn: truly, genuinely,
           really]
    2: by right; "baseball rightfully is the nation's pastime" [syn:
       rightfully, truly]
    3: with sincerity; without pretense; "she praised him sincerely
       for his victory"; "was unfeignedly glad to see his old
       teacher"; "we are truly sorry for the inconvenience" [syn:
       sincerely, unfeignedly, truly] [ant: insincerely]
    4: in fact (used as intensifiers or sentence modifiers); "in
       truth, moral decay hastened the decline of the Roman Empire";
       "really, you shouldn't have done it"; "a truly awful book"
       [syn: in truth, really, truly]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "truly":
   OK, Roger, absolutely, accurately, actually, all right, alright,
   alrighty, amen, and no mistake, as you say, assuredly,
   at all events, at any rate, aye, beyond question, by all means,
   certainly, clearly, confidently, da, de facto, decidedly,
   decisively, definitely, demonstrably, distinctly, doubtlessly,
   duly, easily, exactly, factually, fine, for a certainty,
   for a fact, for certain, for real, for sure, forsooth, genuinely,
   good, good enough, hear, historically, honestly, in actuality,
   in all conscience, in effect, in fact, in reality, in sooth,
   in truth, in very sooth, indeed, indeedy, indubitably, ja, just so,
   justifiably, justly, legitimately, mais oui, manifestly,
   most assuredly, most certainly, naturally, naturellement, nay,
   nothing else but, noticeably, observably, obviously, of a truth,
   of course, okay, oui, patently, positively, precisely, probably,
   properly, quite, rather, really, really-truly, right, rightfully,
   rightly, righto, sensibly, seriously, sincerely, sure, sure thing,
   surely, to a certainty, to be sure, to speak truthfully,
   truthfully, unambiguously, undeniably, undoubtedly, unequivocally,
   unmistakably, unquestionably, veraciously, verily, veritably,
   very well, visibly, well and good, why yes, with truth,
   without doubt, yea, yeah, yep, yes, yes indeed, yes indeedy,
   yes sir, yes sirree