The Collaborative International Dictionary of English v.0.48:

Open-mouthed \O"pen-mouthed`\, a.
   Having the mouth open; gaping; hence, greedy; clamorous.
   --L'Estrange.
   [1913 Webster]

WordNet (r) 3.0 (2006):

openmouthed
    adj 1: with eyes or mouth open in surprise [syn: goggle-eyed,
           openmouthed, popeyed]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "openmouthed":
   advertent, agape, aghast, agog, ajar, alert, all agog, all ears,
   all eyes, amazed, assiduous, astonished, astounded, at gaze,
   attentive, aware, awed, awestruck, bawling, beguiled, bewildered,
   bewitched, blatant, boanergean, boisterous, brawling, breathless,
   burning with curiosity, captivated, careful, clamorous,
   clamoursome, concentrated, confounded, conscious,
   consumed with curiosity, crying, curious, dehiscent, diligent,
   dumbfounded, dumbstruck, earnest, enchanted, enraptured,
   enravished, enthralled, entranced, fascinated, finical, finicking,
   finicky, flabbergasted, gaping, gauping, gazing, ghoulish, gossipy,
   heedful, hypnotized, in awe, in awe of, inquiring, inquisitive,
   intense, intent, intentive, interested, itchy, lost in wonder,
   loudmouthed, marveling, mesmerized, meticulous, mindful, morbid,
   morbidly curious, multivocal, nice, niggling, noisy, observant,
   observing, obstreperous, on the ball, on the job, open-eared,
   open-eyed, oscitant, overcurious, overwhelmed, popeyed, prurient,
   puzzled, quizzical, rapt in wonder, regardful, ringent,
   scopophiliac, screaming, shouting, slack-jawed, spellbound,
   staggered, staring, strident, stupefied, supercurious, surprised,
   thunderstruck, under a charm, vociferant, vociferating, vociferous,
   voyeuristic, watchful, wide-eyed, wonder-struck, wondering,
   yammering, yapping, yawning, yelling, yelping