GatherContent is an awesome tool for planning, structuring and collaborating on web content (they have a short video demo).

They have a read-only API, but only a couple of implementations, so here is mine, in Ruby.

The API is read-only and HTTP POST-based, and so I use HTTParty to manage that side of things, and method_missing to emulate the relevant API calls, and then Hashie to convert the returned Hash into an object. It’s very simple, so give it a try.