There's a new Merlin commentfic community on the horizon - tavern_tales. They're still hashing out the details, but the basic idea is that a new prompt will be posted at the beginning of every month, and people can respond with the fanwork of their choice in the comments. It's open to all pairings and genres, too, so really, there will be something for everyone. Plus, no deadlines which is always good, lol.
I signed up as soon as I could. Commentfic is hard, but so much fun to do.