Nail Knot Tying Tool
If you've ever tried tying a nail knot without a nail knot tying tool, you'll know it's painful, and you probably just take your line down to the fly shop when you need a new leader.
Save some time and hassle, because these tools really do make the job easy. Stick it on your fly vest as it doubles as an eye clearer, and it kind of looks like a fish, if you squint. A lot.