I suppose the most useful thing of all is my fingers/hand. I know it's rather obvious but I just use my hands so much.
While my right hand may do most of the painting my left hand acts as my trusty pallet. You can always tell when I've been painting because my left hand and arm are covered in paint. I never did like using a proper pallet. For some reason I find that I can leave just the right amount of paint on my brush when I use my arm, my paint is the perfect consistency, and my pallet is never out of reach.
My fingers smudge glue into place. I am so damned impatient when it comes to glue. It kills me to wait for the stuff to dry and there have been many times when I have looked up to find that something has slid out of place because of my impatience. I have found that if I put a tiny dab where I need it and then use my finger to make a smooth thin layer, things are dry in no time at all!
My fingers also smudge paint and pencil. Sometimes there is no tool as precise as your finger. It will go exactly where you want it to and apply as much pressure as you demand. Sometimes that extra brush or tool just gets in the way.
For smudging, picking, measuring, deciphering angles, scratching, holding and drawing my hands and fingers really are my greatest artist tools.
