I've thought about participating, but I've been too lazy. I did start unofficially one year, but gave up after a few days. On the other hand if I don't give it a shot this year, I may never do it. And I want to write a novel.
If you are thinking of giving it a try you also only have a day to decide. Here's the link: http://nanowrimo.org
Update: Apparently, I'd signed up for an account in a previous year. I'd forgot. I reset my password and I had signed up four years ago.