You can sign up for a free account at nanowrimo.org, where you can keep track of your word count, access resources, and befriend other writers.
(image courtesy of National Novel Writing Month)
The NaNo Website
The easiest way of keeping track of your words is through the NaNoWriMo website itself. You can sign up for a free account and update your word count each day (or multiple times a day).
WriteTrack is a free tool designed by David Gale that lets you keep track of your words. It's also customizable to your schedule by giving you the option of weighting days differently. If you know you get a lot of writing done on Sundays, you can weight Sundays more heavily so the tracker will know you expect more words. And if there are days you don't expect to get anything done, like maybe Thanksgiving, you can plug that in, too. You need to register to use it.
Pacemaker Planner is similar to WriteTrack. You have to create an account, and with the free version, you can only save two plans. Like WriteTrack, Pacemaker gives you the option to choose days you think you will get more words or fewer words written so you can hit 50,000 at your own pace instead of a steady 1,667 words per day.
Gumberg has the following books on creative writing (and many more!) available: