Recently I found a thread over on britishblades.com about your four favorite bushcraft items. Of course, a knife, firesteel and axe or saw were on nearly everyone's list. A hank of 550 cords would also be up there.
But my choice might be a bandana.
There are hundreds of uses for the little 20" square of cotton (sometimes the only cotton I take with me into the woods). I am reminded of my earliest "bushcraft" book, one I have possibly read more than any other book, the Boy Scout Manual. Scouts around the world typically have a bandana around their neck. I think I prefer a square bandana to the scout's triangle, but either way, the uses are only limited by your imagination and the situation.
Sling, sunshade, waterfilter, mosquito net while sleeping, dishcloth, headband, napkin, tourniquet, tie your hair back ( for those of the long hair persuasion), blow your nose (hankie), "lunchbox", evaporative cooler (roll it up, dunk it in water, tie it around your neck. It will cool your jugular veins and carotid ateries nicely), travel identification (give everyone in your group the shame color and waer them, or tie one on your luggage), placemat, camera lens or sunglass cleaner, tie extra stuff to your backpack, a "Punky Brewster" bracelette, carry foraged goods (mushrooms, berries, shells, rocks....), dust mask (old West style), sweat wiper, tear soaker, belt (I might need more than one), bandage, washcloth, potholder, sleep blindfold (my secret to travel, along with some earplugs, I sleep like a bay anywhere), fly swatter, gift wrap, head and neck sun protection (try tucking it under your hat, Lawrence of Arabia style), dog coller, ice compress, mark a trail, use as a "terrible towel" to chear at a sporting event, signal, water filter, neck gaiter for cold mornings, and possibly my favorite one I found while browsing the internet.....
"disguise your voice on the phone."