Any Writing pads for Macs

I'm looking for a writing pad that I can hookup to my MacBook Pro. So anything I write on this pad it gets written on the screen. For example, I'm doing a presentation using Keynotes or PowerPoint, I would like to be able to write. Doesn't write it on the pad but projects it on the monitor. Not sure if I'm making any sense but if you can direct me to a vender or product that has such a think and it is compatible with my laptop.

Thanks!
Gilbert