This tool allows me to take my teaching to a whole new level. I use it a lot when teaching math. Paper allows me to easily color code items, copy/paste, and has a variety of tools at my fingertips (well, stylustip). Below are just a couple of examples of how I use the app. It's great to take notes in meetings and allows for easy sketchnoting.
I credit Matt Miller (@jmattmiller) for leading me to Paper. If you don't follow Matt on Twitter, I suggest you change that immediately. He's a fabulous resource!
I will be honest to say that there was a small cost to set it up in my classroom, basically because I purchased Airserver to stream my iPad. It was minimal, though, and totally worth it. Try it out for yourself!