Top 1 or 2 most useful tool on a reloading bench.
It'll tell you where you've got contact and where you don't on both a case and the projectile to help get your sizing and seating dies set up properly.
You can label plastic ammo boxes with as much info as you want, it'll stay, and a little rubbing alcohol will take it right off if you need to make a change. I find that especially handy for keeping track of the number of times the brass in the box has been loaded/fired.
I use lots of ziplocks to keep track of variables when working on loads. Powder charges, seating depth changes, brass brand etc. I also use them for storage and the sharpie is a great way to keep track of firings, what rifle the ammo is sized for, etc.
I've occasionally been known to use a sharpie to write a quick "dope chart" right onto a rifle stock. Again, rubbing alcohol will take it right off.