OpenSocial is a set of common Application Programming Interfaces (APIs) for web-based social network applications, developed by Google and a number of other social networks. Applications implementing the OpenSocial APIs will be interoperable with any social network system that supports them. It was released November 1, 2007.
See resources page for more information.
Apache Shindig implements OpenSocial API spec. 0.8.1 and 0.9.
Apache Shindig project actually supports both Java and PHP version. Other Opensocial container implementations exist.
No official XSDs exist yet but you could find unofficial XSDs in the Opensocial resources project:
Note: for performance reasons, Apache Shindig will not validate files against these XSDs at runtime.