Mac OS X: Metanota is a beautiful syncing notes app that works with the wonderful Simplenote service. While it's far from the only option available, it offers excellent organisational features, a three-pane (widescreen-friendly) interface, and the simplicity you need to focus on your writing.
Metanota is designed to be a very simple, plain text note-taking app that syncs with the cloud (or, more specifically, Simplenote), but it still manages to integrate several useful features. You get three panes in the interface. The left pane shows your folders and tags so you can organise notes. The middle pane shows the list of notes associated with that folder, tag or all of them. The right pane displays the note that you're currently viewing. It's very similar to the interface you'll find in a desktop email client. In addition to these easy viewing options, you can star your favourite notes and make them appear at the top of the list for easy access. This is my favourite feature, and wish I could do this in my email client (Sparrow) as well. Additionally, you can sync your notes every 20 seconds, email any note with a click, and search for any text within a specific note.
These features are particularly noteworthy because our current favourite syncing note-taker, Notational Velocity, doesn't offer them. While it's still the fastest way to manage your notes, if you're looking for a more feature-rich option with better organisation, you'll want to check out Metanota. It's a free app, but if you want to remove ads you can pay $4.49 to do so.
Metanota [Mac App Store]