Ubiquitous Capture and The Steathy Pen
One of the main tenants of the GTD methodology is the idea of ubiquitous capture. That is, a way to always get that stuff you have floating around your noggin out of it and into a trusted system that will allow you to evaluate it later. For many, having a pen to write stuff down with at constant disposal is the easiest, quickest and most cost effective solution for this. Problem is, most pens are better designed for the shirt pocket than, say, any other pocket that you more regularly have available in this increasingly casual society.
That being said, here is a rundown of some stealthy pens that can fit in all manner of other places so that you are never without a way to jot down that next great idea (or just an addition to your honey-do list):