The Collaborative International Dictionary of English v.0.48:

doohickey \doo"hick*ey\, n.
   Any object, usually a tool or other device, whose name is
   forgotten, or not known.

   Syn: thingumbob, whatchamacallit, whachamacallit. [PJC]

WordNet (r) 3.0 (2006):

doohickey
    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]