What is the most useful tool you ever made?

Very clever.  Specialized knowledge in a number of different areas came together in this example.  Of course the sales people will probably shudder the next time you drop by the car lot.   

A stroke of genius...

Cool. So this probe is measuring the electric field from the demister elements? (Or is it actually contacting resistive/conductive elements?)

It measures the voltage at any point on the 'element' which, in most cases, is a horizontal stripe across the rear window. My car has seven of them. All the left ends are joined by one slightly larger vertical stripe which is connected to the +12v supply from the battery via the dashboard switch. Similarly, all the right ends are connected by a second vertical stripe to 0v, a.k.a. the battery negative terminal, the chassis, an unpainted and non-rusty bolt head, etc. The meter negative terminal is connected to all the right ends of the demister elements via the car metalwork, the meter positive terminal 'connects' to each demister element in turn by the user holding the probe and pressing it very gently into contact with the element so that the conductive foam deforms very slightly and becomes a low-value resistor in series between the element and the copper tube. The foam is for protection of the element rather than for any electrical reason.

From now on, 'track' refers to all the elements in parallel connected between the +12v and the 0v vertical stripes, and 'probe' and 'slider' are synonymous.

Basically, what I've done is to construct a potentiometer (usually pronounced 'pot') such that one end of the track is connected indirectly (via dashboard switches etc.) to the +12v battery terminal, and the other end of the track to the negative battery terminal (via car bodywork etc.) The slider can be positioned anywhere along any one of the elements by the user, and a good element will cause the meter to display anything between +12v at the left-hand end and 0v at the right-hand end. A dodgy element will cause a sudden transition from 0v to 12v as the probe is moved across a break from right to left and vice versa.

N.B. Other vehicles may have different demister configurations and different ways of switching them on and off. Check the wiring diagram first if it is available, or use the DVM with both its' norrmal probes and 15VDC selected to verify voltages before testing. To prevent damage do not touch any of the elements or either of the vertical stripes with the pointy end of a meter probe.

I am surprised the grid is not coated with insulation. I would have thought any condensation on the window would short out the grid if the surface was conductive.

Good question Doug.

The best tool that I have ever made is most likely the light ring that I built for my microscope that I use to assembly PCBs.  I built this over 15 years ago and it was gone through a couple of re-design cycles (on the control side).  It is used frequently and has performed flawlessly all this time.

I created a software tool that could measure all kinds of information about computer architectures, primarily for embedded avionics.

I ended up using the model for many projects and generated about $10 Million USD worth of business on several projects over about 15 years.

Great solution. I also made LED light rings for my microscopes with about 15 years between the 2 designs.

Wow, yeah software tools are a whole other branch of tool types.

And the shadow this ringlight produces is a thing of beauty.