Supernatural 25 (spn_25) has a series of Theme Sets for claiming, where you write 25 stories for a different theme set.
It works very much like fanfic100 or prisonbreak100, except that there are only 25 stories to write, multiple theme sets, and several people can claim one table at a time.
What's not to love?
Plus, there are two additional tables meant to inspire those of you who love to write Wincest, smut, porn, and the like for Supernatural.
Check it out, look over the prompt tables, and stake your claim!