From WordNet (r) 2.1 (2005) (link)

  dumbfounded
      adj 1: as if struck dumb with astonishment and surprise; "a
             circle of policement stood dumbfounded by her denial of
             having seen the accident"; "the flabbergasted aldermen
             were speechless"; "was thunderstruck by the news of his
             promotion" [syn: dumbfounded, dumfounded,
             flabbergasted, stupefied, thunderstruck,
             dumbstruck, dumbstricken]

From Collins COBUILD Advanced Learner's English Dictionary (link)

  dumbfounded
    If you are dumbfounded, you are extremely surprised by something.
    I stood there dumbfounded.
   = astonished
   ADJ: usu v-link ADJ
  
  
  

From English Explanatory Dictionary (Synonyms) (link)

  dumbfounded
  adj. dumfounded, amazed, shocked, surprised, startled, astonished, astounded, bewildered,
  staggered, floored, nonplussed, overwhelmed, speechless, stunned, thunderstruck, dazzled, dazed,
  dumbstruck, taken aback, confused, confounded, bemused, perplexed, baffled, disconcerted, Colloq
  bowled over, flabbergasted, knocked out, thrown (off), US thrown for a loss, Brit knocked for
  six, knocked sideways: She is dumbfounded that he proposed marriage.
  
  
  

From Moby Thesaurus II by Grady Ward, 1.0 (link)

  78 Moby Thesaurus words for "dumbfounded":
     agape, aghast, agog, all agog, amazed, anaudic, aphasic, aphonic,
     astonished, astounded, at gaze, awed, awestruck, baffled, beguiled,
     bemused, bewildered, bewitched, breathless, captivated, confounded,
     confused, dazed, dazzled, disconcerted, dismayed, dumb,
     dumbstricken, dumbstruck, enchanted, enraptured, enravished,
     enthralled, entranced, fascinated, flabbergasted, floored, gaping,
     gauping, gazing, hypnotized, in awe, in awe of, inarticulate,
     knocked out, lost in wonder, marveling, mesmerized, mum, mute,
     nonplussed, open-eyed, openmouthed, overwhelmed, perplexed,
     popeyed, puzzled, rapt in wonder, shocked, speechless, spellbound,
     staggered, staring, startled, stricken dumb, stunned, stupefied,
     surprised, taken aback, thunderstruck, tongue-tied, tongueless,
     under a charm, voiceless, wide-eyed, wonder-struck, wondering,
     wordless