Announced in my presentation at EEUK 2013: http://www.putyourlightson.net/news/eeuk-2013-presentation
Open API is a front-end API for ExpressionEngine that provides authentication and CRUD functionality to the content and data in the CMS.
Full add-on documentation: http://www.putyourlightson.net/open-api/docs
API documentation: http://docs.eeopenapi.apiary.io/
* Please note that Open API is in beta - use in production at your own risk.