Scrivener is great. For those who work in the Windows environment, a Windows version is in beta development. There's a free download of the latest build (0.2.4) available here:
http://www.literatureandlatte.com/scrivenerforwindows/
They expect to have a release version available in July 2011 but the beta is already pretty stable.
I might add, there is a new book on Amazon called
Writing a Novel with Scrivener. It gives one author's advice on how he uses the program to write novels. Scrivener works well to write short stories and non-fiction as well.