WordNet sense 1 (something unspecified whose name is either forgotten or not known):

doodad, doohickey, gimmick, gismo, thingamabob, thingamajig, whatchamacallit, widget, gubbins, thingmabob, thingmajig, thingumabob, thingumajig, thingummy, whatsis, doojigger, whatchamacallum

WordNet sense 2 (a device or control that is very useful for a particular job):

appliance, contraption, contrivance, convenience, gadget