Here's some definitions you might find useful...
Widget- A simple object without movable parts. Usually made of plastic(bottle opener)
Doodad/Doohickey- usually a part of a larger contraption. A shear pin would be a good example.
Thingamajig/Thingamabob- Similar to a widget, but heavier duty, and more practical(universal adapter for a socket wrench)
Contraption- The paragon of the engineering discipline. The useless box is a perfect example.
Whatchamacallit- A candy bar as stated above :^D