Friction Hitch Loop
One of the cheapest most versatile items in your outdoor climbing kit. You can make one out of a nylon sling, but having a dedicated compact friction loop ready to go is must in the outdoor environment. Having two on you at all times is common and solves a lot of problems.
Here are a few uses:
Adjustable connection point between safety line and personal tether
“Third hand” backup for rappelling
Progress capture in a hauling system
Rope ascension progress capture
Backup for lowering with a Munter Hitch or tube style belay device when top managed belaying