This is my first year doing Nano. I have my characters more-or-less fleshed out, and I have a premise for the story, but I don't really know where it will go after the first few scenes. Hahah.
The guy who started Nanowrimo wrote a book called No Plot? No Problem!, and one of the things he suggests doing is making two lists: one for things you like in stories and would be good to include, and one for things you dislike in stories and would like to avoid including in yours. So I did that in Evernote. I also have, as aforementioned, plenty of visual references - one for the setting/environment, and one for each character. (I like to be able to visualize my characters doing stuff, and plus I've found some really, really good pictures that totally capture them.)
One of the most recent blog posts from the Nanowrimo team made the point that it's important to write down your inspiration the minute it strikes, so that's where a mobile version of Evernote comes in handy.