http://writeordie.drwicked.com/
You enter how many words you want to write and set a time limit - say, 500 words in 20 minutes. You write. While you're writing, a progress bar shows how close you are to reaching your goal; you also have a timer and an ongoing word count on the lower part of the text box. If you stop writing, the software makes noises at you, prodding you to continue.
What's especially unique is that you can have "writing wars" with other users. You and another writer agree on a certain number of words in a certain period of time, then you both start writing. You can see your "competitor's" progress bar as well as your own. Whoever reaches the word count first wins!
Oh, and when you meet your word count within the established time frame, there's a little trumpet-of-victory sound that's kind of a nice boost to the ego.

You can use it for free on the website or pay ten bucks for your own desktop version. I got the desktop version (using money C-C just put in my PayPal) and I think it may be the best ten bucks I ever spent in my life.
A request came in to C-C tonight for a 300 word article on the basics of a healthy lifestyle. I got on Write or Die, cranked out a first draft in about 6 minutes, saved the draft to Word, edited and tweaked for another 5 minutes, and submitted. So if it's accepted that'll be 13 bucks in the bank for a total of maybe fifteen minutes' work!
Check it out, guys! I want to have a writing war with someone!