My rules for a kitchen tool are simple:
It must be less work than doing it before;
It must be easier to do than before;
The cleanup must not outweigh the benefits.
Here are a few things that I actually do find useful, that might be a little different from the norm.
Flat edged wooden spoon
Not really a spoon as such, but not quite a spatula. Like most people, I used wooden spoons for years. Even in culinary school we used them to an extent. It wasn't until recently I realized how dumb this was.
Think about this: you're stirring the pot to make sure whatever you are cooking doesn't burn. A wooden spoon makes a few millimetres of contact with the pan. That means lots of stirring and scraping to try and keep the food from sticking.
Now use a flat bottomed one. You get three or four inches of contact. Massive amounts of food or liquid move with your spoon. It's not any harder to stir, but it takes much less stirring to get the same results as a wooden spoon and its just as easy to get into corners.
The cup part of a wooden spoon is fairly useless as well. You aren't serving with it, tasting from it tastes mostly like wood, and it doesn't give you any advantages over a flat surface.
Wooden spoons have a place for a few applications, like stirring pasta water (you'll splash hot water with the flat kind), but its a secondary role in my kitchen.
This is a single use tool that has a special place in my heart. Though the attachments take up space, I find it useful enough to not mind.
You may wonder why I have one, when ground meat is so easy to find. It's true, it is, but outside of ground beef it can be twice as expensive. Just look at the price per kilogram of ground chicken sometime. A 10 second job should not add that much to the price.
With my own grinder, I can grind any type of meat quickly and easily. Ground chicken and pork is great for home made chinese food. If you buy good beef on sale like sirloin or chuck, you can get great tasting burgers too.
I find it tastes better too. In the store, most ground meat is made up from leftover bits from breaking down the animals. The whole pieces you buy tend to be of a better quality.
Cleaning is easy, just scrub in hot dishwater and put the pieces in a mild bleach solution. Rinse off and you're ready for the next batch.
I use the grinder attachment for a KitchenAid mixer to grind my own meat, however other mixers have grinder attachments as well. In a pinch, a food processor can be used for small batches.
Strange but true, I have a toothbrush near my sink for cleanup. It's great for getting into nooks and crannies (see meat grinder above), and is easy to maneuver. It's also great for getting stuck food out from the hilt and the blade of your knife. Look close and you may see a thin black line where the metal meets the handle. That's food residue. I doubt it will hurt you, but it's nice to get rid of it and very easy to do with a toothbrush.
I would suggest buying a new one for your kitchen however, instead of using an old one from the bathroom. A cheap, bristle only, one is fine, and you'll avoid the "ew" factor of using a used one.