Sifting flour.
Colanders and slotted spoons can be used to make spaetzle.
When I want to dice an apple or a pear, I cut it in half and use a melon baller to remove the core.
My strawberry huller is single-use.
My wife has a liquid level detector (called a “Say When”) that she hangs over the lip of her cup or glass before she pours her drink. When the liquid approaches the rim, it completes a circuit with two metal electrodes, and a buzzer sounds off.
I have a gnocchi board - small (about 2x3") grooved, made out of wood, used for hand rolling gnocchi. Can’t think of a single other use for it. Also have a pasta measure (metal with 3 holes in it to measure 2, 3 and 4 serves of dry pasta).
Then there’s the olive pitter, but we use that for cherries as well, so that’s dual use.
Alton Brown said on “Good Eats” the the only uni-tasker he had in his kitchen was the fire extinguisher.
That thing you use for cutting butter/lard/fat into flour for pastry.
Pickle fork
Sushi mat
I had a small can punch that I had to send away to the canned milk company for that was for opening cans of evaporated milk, but it seems to have disappeared. Never used it for anything else.
Just about everything else in my kitchen gets pressed into service for whatever is needed. I have way too much kitchen stuff, but there’s always room for something else useful.
Good one.
A cork screw seems fairly single use. I have one of those ‘rabbit’ types and can’t imagine using it for anything else.
Can also be used for shredding cabbage.
For cooking purpose corkscrews seem pretty limited, though I’m wondering if maybe one could be used to open clam and oyster shells. I might give that a try at the next opportunity.
Maybe a bagel holder for slicing it?
This is what I came in to post. Of course the one time I actually have corn on the cob I can’t find the blasted holders.
??
It would be more useful if it worked the other way - alerting you when something has boiled down to a certain depth.
Mrs. K has vision problems, IIRC. But your idea would be something I would buy.
You could use that as a rolling pin. Or a meat tenderizer. Or for chasing away unwanted guests.
Egg separators are the first thing that came to mind.
But fingers have lots of other uses?!?! 
My mom had a ravioli rolling pin like this
Pie crust shields. I’m not sure there’s much else you’d use them for.
Pulling hard boiled eggs out of the water.
That’s what egg tongs are for. 