Scrivener

Scrivener (Literature and Latte for Mac OS X) is a wonderful tool for your book or ebook project. I have it on my Mac and was impressed with how quickly I could see my projects at a glance and each of the components therein. The site has solid video tutorials and the software developer, Keith (KB), runs a forum on the site and is responsive to questions (remember to look through this awesome resource prior to asking a question though). You can check it out for a free trial period and if you like it pick it up for $45 USD. If you poke around the NaNoWriMo site though, you might find a coupon that'll save you $10 or so (at least I did when I picked up my copy).

Reporter's Desktop

Here's a resourse that's been around for a long time, the Reporter's Desktop.  It hasn't been updated in a year but it is still good to go. Adding it to the writer's resource section of this site today.