I signed up to the Comic-Collab Project on Tumblr, a comic collaboration involving Dean/Cas (and other characters if the situation presents itself of course). The mod, Wondersmith draws the first page of a comic and the artist that follows has 10 days to draw the next page and so on and so forth until the end of the sign-up list.
Last date to sign up: Saturday, November 26; 5:00pm GMT.
Challenge starts on: Monday, November 28; (no set time, date might change)
How to sign up: This challenge takes place on Tumblr. All you have to do is to post on your Tumblr a text post with the title "I'M IN" and to tag it as destiel comic collab.
That's really a great idea! If someone could organize the same kind of thing for Sam/Dean, that would be just awesome!