Plot Storming is a new community geared towards helping writers and GMs brainstorm, develop and workshop plots. From their intro:

Suffering from a creative block? Enjoy plot development or world building? Have a skeleton, but need the meat? Love swapping ideas with other writers? Then Plot Storming is for you! Bring your ideas, characters, plots, and worlds, and we’ll “storm” it. We enjoy helping develop and refine ideas so writers and gamers can create a better story and world.

It’s a very young site, but it looks promising. I particularly like their Synergy program, which allows members to have their own sub-forums set up for group creative projects.

(Via TT forum member KeshFerrar‘s signature. As an aside, if you run a site that’s helpful to GMs, don’t hesitate to tell me about it — I try to ferret out links like this one, but I’m bound to miss at least a few!)