yuletide isn't open for sign-ups yet, however if you're considering it, THIS POST gives all the details you need to know.
Meanwhile, over at alias_slash, jenab has announced an October fic challenge.
House Fest is taking prompt submissions, so now is the time to unload the bunnies you've been nurturing, but just don't have time to write yourself.
For the House/Cameron contingent, there's a new challenge up at the H_C_Smutathon I think that's a "members only" community, so if you're interested, you would need to join. The challenges always produce some seriously hot smut.
And since it's my LJ, let me take one more chance to pimp the Halloween Party and thank everybody who's RSVP'd and pimped it. It looks like we're gonna have a tremendous turnout and a real diversity of fandoms represented.