I'm playing around with some different writing tools at the moment. Specifically, Scrivener and Writeroom.
Scrivener is an extremely powerful package with all kinds of features like storyboarding and management tools for large writing projects.
Writeroom is an absolutely basic, full-screen text editor. No bells or whistles (or animated paperclips) of any kind.
I am a notorious goldfish when it comes to focussing my attention on any one thing at a time so I am finding a full-screen editor to be quite useful in limiting my distractions (Scrivener has a full-screen mode as well I just noticed but I haven't used it much yet).
I suspect Scrivener would prove to be more useful if you were doing any kind of critical or technical writing where you needed to frequently refer to sources of some kind whereas Writeroom would shine in a purely creative environment where you weren't interested in anything other than getting the ideas on to the page.
I'll write an update when I've used them both a bit more.