An immersion chiller cools hot liquids quickly by running cold water through a coiled copper pipe that's immersed in the liquid. You can make this DIY model to whatever size fits your needs.
If you make large batches of stock, broth, brine, or tea, you know how long it can take to cool down before you put it in your fridge. You don't want to refrigerate it hot because that much hot liquid can raise the temperature inside. And you don't want to leave stock out long enough to cool first because you don't want to encourage bacteria growth.
An immersion chiller cools litres of liquid in just a few minutes. The build does take some effort. You'll need to heat the pipe (the kind of copper pipe intended for plumbing) using a propane torch or other heat source so that it's pliable enough to bend. You'll then wrap the pipe in a coil around a form (like a paint can) that's the size you want. When the coil is done, you just attach the hoses and it's ready to use.
You can buy immersion chillers intended for beer brewers, but they tend to be pretty large, typically made to cool a few dozen litres. Making your own means that you can make it exactly the size you need to fit in your favourite pot and use right in your kitchen. And if you're worried about wasting water, just collect it on the other end and reuse it for whatever you want. It's only come in contact with the inside of a clean copper pipe.
Hit up the full article for a step-by-step rundown.