I started to write a comment in Sean's post on outlines, but it quickly turned into Monty Python's Spanish Inquisition sketch—my chief use of outline is as a book writing tool...book writing tool and book selling tool..selling and writing... My two main uses for an outline are book writing and book selling...and structuring. My three uses are... and so on. Now it's become so big that I'm actually going to do it as a two or three part front page post.
Over the years I've become a militant outliner. My first two books were written off the cuff, and though I still love the bones of both stories, I can see how knowing where I was going from the beginning would have produced a better end product. My third had a crude outline, and my fourth had a cruder one. Since then, I've gotten steadily more efficient and focused with outlines and it's led to big improvements in writing speed and quality.
Brief digression: I hated outlining in college. Absolutely hated it. I would go to great lengths not to have to write outlines for papers, even going back and writing outlines post paper in classes that required them so that it looked as though I had followed the desired process. To all the professors who tried to get me to outline back then, mea culpa, you were right, I was wrong.
Back to the main topic. I use many different types of outlines in my work (updated to add timeline):
These each have their own foibles and uses, and I'll get into that in part II tomorrow. In the meantime, if you've got comments or questions, I'd love to hear them.