WordNet (r) 3.0 (2006):

flabbergasted
    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:

49 Moby Thesaurus words for "flabbergasted":
   agape, aghast, agog, all agog, amazed, astonished, astounded,
   at gaze, awed, awestruck, beguiled, bewildered, bewitched,
   breathless, captivated, confounded, dumbfounded, dumbstruck,
   enchanted, enraptured, enravished, enthralled, entranced,
   fascinated, gaping, gauping, gazing, hypnotized, in awe, in awe of,
   lost in wonder, marveling, mesmerized, open-eyed, openmouthed,
   overwhelmed, popeyed, puzzled, rapt in wonder, spellbound,
   staggered, staring, stupefied, surprised, thunderstruck,
   under a charm, wide-eyed, wonder-struck, wondering