Email as a Content API
I think email is highly underrated as a content API. I know its not what we traditionally think of as an API, but has a lot of potential as a basic interface into an application.
An example of this is If This Then That (iftt). I use iftt to syndicate content to some of my blog platforms. As part of my blogging strategy I want posts I publish on API Evangelist, to also show up on my Blogger, Posterous, Tumblr and Wordpress.
Using iftt you can setup tasks that get triggered on certain events. One such trigger is a new post to an RSS feed. So when a new blog posts shows up in the API Evangelist RSS feed I can have iftt pull the title and body and push to a channel. iftt provides channels for Posterous and Tumblr, allowing me to automatically syndicate using their APIs.
There are no channels for Blogger and Wordpress, so I use their Gmail channel in conjunction with the post by email features on Blogger and Wordpress. So instead of using an API to syndicate my content, I use email as an interface for these blogging platforms.
Using email as a content API is obviously not for every situation, but for some application processes, it might just work for moving content from one application to another, without much overhead.