The good folks at Storyist are hooking up NaNoWriMo-ers with a free trial of their software for the month of November and 25% off if you choose to buy it. I wanted to try it out before recommending it and am delighted with it so far.
What's good: Drag-and-drop storyboarding, character sheets, and plot lines. A place for random notes. It's flexible so it mimics the way my brain would love to work if it were organized.
What's better: My project is on Dropbox and the license allows for use on multiple machines, so can access it on the MacDaddy and on my laptop.
What's best: After organizing my notes and ideas for my NaNo project, I've discovered plot holes, character inconsistencies, and other minor irritations that could make or break a first draft.