I've been using OmniOutliner every day for the last few weeks. What a great writing tool.
One thing I love about it: I can have multiple columns in the outline. I write the main content in the leftmost column, and write notes in another column... it's just like having a "margin" to scribble in. :-)
It also supports rich text, and since it's a Cocoa application, it has great formatting features and has real-time spell checking.
(Not bad for $21.12)