This year I participated in Novel in a Day, which is a collaboration between writers to complete one chapter of a novel in one day. Afterwards all of the chapters are compiled into the complete novel.
It’s organised by Pigfender on the Scrivener forums Literature and Latte (but you don’t need to use Scrivener to participate–newbies are welcome), and he does a fantastic job! The participants are basically going in blind–you have no idea what you’re writing until you get your brief, and even then you’re given only the barest bones of context so you’re not sure where your part fits in with the rest of the story (although you’re welcome to guess).
If you’re interested, you can get more information about the project here. He only hosts this once a year and this year’s has just passed, but I highly encourage you all to sign up next year! It’s definitely a challenge and I think I aged 10 years in one day, but I would do it again in a heart beat!
You can see past Novels in a Day here, and you can find this year’s novel, Section 7, here. There were enough participants this year to have multiple copies of the same book. I’m in the red version so if you’re only going to check out one copy then check out that one! But feel free to compare both versions. It’s interesting to see the authors’ different takes on the same subject.