Inspired by AtomicStryker's Magic Yarn mod, named after the myth of the labyrinth and Minotaur, Ariadne's Thread adds an item that lets you see where you've been.
Once you've crafted the Thread, simply right-click it to get it to start working. Every 20 ticks, it'll save your position (if you're far away enough from your previous) and draw a line connecting them. This line is visible through blocks, and changes color to show the start and end of your travel.
This is mainly intended to help you find your way back out of sprawling cave systems, though it also works for any similar use-cases.
Pretty much everything in the mod is configurable, as well! You can change the start and end colors, the check interval, the minimum distance between points, etc. You can also set the max amount of locations to save, for performance reasons.