OH! Has everyone seen this
You basically pick a time (like 15 minutes), a number of words (say, 300), and it gives you a little box, and you type until the time is up or you reach your word goal. If you stop typing for a length of time, it will start making noise or something (depending on what level you choose. I think the "kamikaze" level actually deletes your writing).
You can pause the timer, but only once.
It's very fun if you need a bit of a push to get writing - and it works for nano when really the word count is more important than content.