The Campaign Builders’ Guild is a resource site and community for GMs with a simple goal: “…to help with the creation of GM-created, homebrew campaigns.”
The CBG has been around for just over a year, and is still fairly small — but it’s also quite active. The forums are at the heart of the site, with Homebrews and Campaign Elements and Design leading the way. Since the CBG started on the WotC forums, a lot of their discussion is focused on D&D campaigns, but they’re open to all systems and genres.
I’ve never seen a community centered solely around this topic, and I dig the CBG’s tight focus — homebrewing is a rich niche to explore. (Via EN World.)