Thanks to gategrrl, I've just discovered a new writing thing: Scriptfrenzy, which is the scriptwriting equivalent of NaNoWriMo. It runs in much the same way as NaNo, except, as you'd expect given the name, you write a 20,000 word script instead of a novel. The script can be for either stage or screen.
Best of all, though, you can collaborate with a partner! I really love that idea so, so much; I wish we could do that for NaNo.
It all kicks off for the very first time on 1 June 2007. So if scripts are your thing, you can be one of the first to win the Scriptfrenzy badge of honour.