This client-side mod shows a big version of whatever item you hover over.
Great for builders looking for the right materials!
By default you can toggle it on and off with Shift + Z, which can be changed in the Minecraft controls.
There is also an unmapped keybinding you can use if you want to hold a button down to make it display.
You can change the zoom amount in the config or by setting the hotkeys for Zoom In and Zoom Out.