The Collaborative International Dictionary of English v.0.48:

dumbfounded \dumbfounded\ adj.
   1. same as astounded.

   Syn: amazed, astonied, astonished, astounded, flabbergasted,
        stunned, stupefied, thunderstruck.
        [WordNet 1.5]

   2. astonished and confounded.

   Syn: amazed, dumfounded.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

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]

Moby Thesaurus II by Grady Ward, 1.0:

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