A couple of years ago I participated in NaNoWriMo (National Novel Writing Month) during which I wrote a [very] rough draft of a novel that is slowly making its way into a more polished form. It was a terrific exercise, one that I wouldn't mind doing again some November.
And now I hear of ScriptFrenzy, which sounds pretty similar. Participants are hoping to complete a 20,000 word screenplay during the month of June. Now, I've just begun a screenplay, as I noted earlier, and I'm participating in a screenwriting workshop at Tinker Mountain in mid-June, so I think I might as well sign up for this.