NaNoWriMo: Write a novel during National Novel Writing Month
It's a project that began in 1999 in order to provide a structured, supportive group in which to write 50,000 words in a month, which is approximately a 175-page novel. Just 21 people participated in 1999 and by 2010, over 200,000 people took part (though only 30,000 uploaded the complete 50,000 words by the end of the month).
The website describes the project as, "Because of the limited writing window, the ONLY thing that matters in NaNoWriMo is output. It's all about quantity, not quality. This approach forces you to lower your expectations, take risks, and write on the fly. Make no mistake: You will be writing a lot of crap. And that's a good thing. By forcing yourself to write so intensely, you are giving yourself permission to make mistakes. To forgo the endless tweaking and editing and just create. To build without tearing down."
We participated one year. We didn't get to 50,000 words, but it was still really fun. Check out the website for more information.