WordNet (r) 3.0 (2006):

thingumabob
    n 1: something unspecified whose name is either forgotten or not
         known; "she eased the ball-shaped doodad back into its
         socket"; "there may be some great new gizmo around the
         corner that you will want to use" [syn: doodad,
         doohickey, doojigger, gimmick, gizmo, gismo,
         gubbins, thingamabob, thingumabob, thingmabob,
         thingamajig, thingumajig, thingmajig, thingummy,
         whatchamacallit, whatchamacallum, whatsis, widget]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "thingumabob":
   affair, article, artifact, dingus, dofunny, dohickey, dojigger,
   dojiggy, domajig, domajigger, doodad, dowhacky, eppes, etwas,
   flumadiddle, gadget, gigamaree, gimmick, gizmo, hickey, hootenanny,
   hootmalalie, jigger, material thing, object, quelque chose,
   something, thing, thingum, thingumadad, thingumadoodle,
   thingumajig, thingumajigger, thingumaree, thingummy, whatchy,
   widget