You can have an organ or feature of any multicellular organism implanted in your body. What is it, why do you want it and how would you use it?

I'd probably want a tail with features similar to other primates. I'd use it for better balance, and also wag it to demonstrate I'm happy or excited. It would probably need some fur, otherwise it would look hideous. I don't think any tail can really support my whole 90kg adult body, but it would certainly make it easier to climb trees and other stuff. Not that I have a need to climb, but I might give a try if I had a tail.

And it might also come in handy to swing a Brazilian hammock or hold a cold beverage.