WordNet (r) 3.0 (2006):

gadget
    n 1: a device or control that is very useful for a particular
         job [syn: appliance, contraption, contrivance,
         convenience, gadget, gizmo, gismo, widget]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "gadget":
   affair, apparatus, appliance, article, artifact, business, concern,
   contraption, contrivance, creation, device, dingus, dofunny,
   dohickey, dojigger, dojiggy, domajig, domajigger, doodad, dowhacky,
   eppes, etwas, flumadiddle, gigamaree, gimcrack, gimmick, gizmo,
   hand tool, hickey, hootenanny, hootmalalie, implement, instrument,
   invention, jigger, machine, material thing, means,
   mechanical device, mechanism, object, power tool, precision tool,
   quelque chose, something, speed tool, thing, thingum, thingumabob,
   thingumadad, thingumadoodle, thingumajig, thingumajigger,
   thingumaree, thingummy, tool, utensil, whatchy, widget